virt/kvm/pfncache.c
// SPDX-License-Identifier: GPL-2.0-only
/*
* Kernel-based Virtual Machine driver for Linux
*
* This module enables kernel and guest-mode vCPU access to guest physical
* memory with suitable invalidation mechanisms.
*
* Copyright © 2021 Amazon.com, Inc. or its affiliates.
*
* Authors:
* David Woodhouse <dwmw2@infradead.org>
*/
#include <linux/kvm_host.h>
#include <linux/kvm.h>
#include <linux/highmem.h>
#include <linux/module.h>
#include <linux/errno.h>
#include "kvm_mm.h"
/*
* MMU notifier 'invalidate_range_start' hook.
*/
void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm, unsigned long start,
unsigned long end)
{
struct gfn_to_pfn_cache *gpc;
spin_lock(&kvm->gpc_lock);
list_for_each_entry(gpc, &kvm->gpc_list, list) {
read_lock_irq(&gpc->lock);
/* Only a single page so no need to care about length */
if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
gpc->uhva >= start && gpc->uhva < end) {
read_unlock_irq(&gpc->lock);
/*
* There is a small window here where the cache could
* be modified, and invalidation would no longer be
* necessary. Hence check again whether invalidation
* is still necessary once the write lock has been
* acquired.
*/
write_lock_irq(&gpc->lock);
if (gpc->valid && !is_error_noslot_pfn(gpc->pfn) &&
gpc->uhva >= start && gpc->uhva < end)
gpc->valid = false;
write_unlock_irq(&gpc->lock);
continue;
}
read_unlock_irq(&gpc->lock);
}
spin_unlock(&kvm->gpc_lock);
}
static bool kvm_gpc_is_valid_len(gpa_t gpa, unsigned long uhva,
unsigned long len)
{
unsigned long offset = kvm_is_error_gpa(gpa) ? offset_in_page(uhva) :
offset_in_page(gpa);
/*
* The cached access must fit within a single page. The 'len' argument
* to activate() and refresh() exists only to enforce that.
*/
return offset + len <= PAGE_SIZE;
}
bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
{
struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
if (!gpc->active)
return false;
/*
* If the page was cached from a memslot, make sure the memslots have
* not been re-configured.
*/
if (!kvm_is_error_gpa(gpc->gpa) && gpc->generation != slots->generation)
return false;
if (kvm_is_error_hva(gpc->uhva))
return false;
if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len))
return false;
if (!gpc->valid)
return false;
return true;
}
static void *gpc_map(kvm_pfn_t pfn)
{
if (pfn_valid(pfn))
return kmap(pfn_to_page(pfn));
#ifdef CONFIG_HAS_IOMEM
return memremap(pfn_to_hpa(pfn), PAGE_SIZE, MEMREMAP_WB);
#else
return NULL;
#endif
}
static void gpc_unmap(kvm_pfn_t pfn, void *khva)
{
/* Unmap the old pfn/page if it was mapped before. */
if (is_error_noslot_pfn(pfn) || !khva)
return;
if (pfn_valid(pfn)) {
kunmap(pfn_to_page(pfn));
return;
}
#ifdef CONFIG_HAS_IOMEM
memunmap(khva);
#endif
}
static inline bool mmu_notifier_retry_cache(struct kvm *kvm, unsigned long mmu_seq)
{
/*
* mn_active_invalidate_count acts for all intents and purposes
* like mmu_invalidate_in_progress here; but the latter cannot
* be used here because the invalidation of caches in the
* mmu_notifier event occurs _before_ mmu_invalidate_in_progress
* is elevated.
*
* Note, it does not matter that mn_active_invalidate_count
* is not protected by gpc->lock. It is guaranteed to
* be elevated before the mmu_notifier acquires gpc->lock, and
* isn't dropped until after mmu_invalidate_seq is updated.
*/
if (kvm->mn_active_invalidate_count)
return true;
/*
* Ensure mn_active_invalidate_count is read before
* mmu_invalidate_seq. This pairs with the smp_wmb() in
* mmu_notifier_invalidate_range_end() to guarantee either the
* old (non-zero) value of mn_active_invalidate_count or the
* new (incremented) value of mmu_invalidate_seq is observed.
*/
smp_rmb();
return kvm->mmu_invalidate_seq != mmu_seq;
}
static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
{
/* Note, the new page offset may be different than the old! */
void *old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
kvm_pfn_t new_pfn = KVM_PFN_ERR_FAULT;
void *new_khva = NULL;
unsigned long mmu_seq;
lockdep_assert_held(&gpc->refresh_lock);
lockdep_assert_held_write(&gpc->lock);
/*
* Invalidate the cache prior to dropping gpc->lock, the gpa=>uhva
* assets have already been updated and so a concurrent check() from a
* different task may not fail the gpa/uhva/generation checks.
*/
gpc->valid = false;
do {
mmu_seq = gpc->kvm->mmu_invalidate_seq;
smp_rmb();
write_unlock_irq(&gpc->lock);
/*
* If the previous iteration "failed" due to an mmu_notifier
* event, release the pfn and unmap the kernel virtual address
* from the previous attempt. Unmapping might sleep, so this
* needs to be done after dropping the lock. Opportunistically
* check for resched while the lock isn't held.
*/
if (new_pfn != KVM_PFN_ERR_FAULT) {
/*
* Keep the mapping if the previous iteration reused
* the existing mapping and didn't create a new one.
*/
if (new_khva != old_khva)
gpc_unmap(new_pfn, new_khva);
kvm_release_pfn_clean(new_pfn);
cond_resched();
}
/* We always request a writeable mapping */
new_pfn = hva_to_pfn(gpc->uhva, false, false, NULL, true, NULL);
if (is_error_noslot_pfn(new_pfn))
goto out_error;
/*
* Obtain a new kernel mapping if KVM itself will access the
* pfn. Note, kmap() and memremap() can both sleep, so this
* too must be done outside of gpc->lock!
*/
if (new_pfn == gpc->pfn)
new_khva = old_khva;
else
new_khva = gpc_map(new_pfn);
if (!new_khva) {
kvm_release_pfn_clean(new_pfn);
goto out_error;
}
write_lock_irq(&gpc->lock);
/*
* Other tasks must wait for _this_ refresh to complete before
* attempting to refresh.
*/
WARN_ON_ONCE(gpc->valid);
} while (mmu_notifier_retry_cache(gpc->kvm, mmu_seq));
gpc->valid = true;
gpc->pfn = new_pfn;
gpc->khva = new_khva + offset_in_page(gpc->uhva);
/*
* Put the reference to the _new_ pfn. The pfn is now tracked by the
* cache and can be safely migrated, swapped, etc... as the cache will
* invalidate any mappings in response to relevant mmu_notifier events.
*/
kvm_release_pfn_clean(new_pfn);
return 0;
out_error:
write_lock_irq(&gpc->lock);
return -EFAULT;
}
static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva)
{
unsigned long page_offset;
bool unmap_old = false;
unsigned long old_uhva;
kvm_pfn_t old_pfn;
bool hva_change = false;
void *old_khva;
int ret;
/* Either gpa or uhva must be valid, but not both */
if (WARN_ON_ONCE(kvm_is_error_gpa(gpa) == kvm_is_error_hva(uhva)))
return -EINVAL;
lockdep_assert_held(&gpc->refresh_lock);
write_lock_irq(&gpc->lock);
if (!gpc->active) {
ret = -EINVAL;
goto out_unlock;
}
old_pfn = gpc->pfn;
old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
old_uhva = PAGE_ALIGN_DOWN(gpc->uhva);
if (kvm_is_error_gpa(gpa)) {
page_offset = offset_in_page(uhva);
gpc->gpa = INVALID_GPA;
gpc->memslot = NULL;
gpc->uhva = PAGE_ALIGN_DOWN(uhva);
if (gpc->uhva != old_uhva)
hva_change = true;
} else {
struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
page_offset = offset_in_page(gpa);
if (gpc->gpa != gpa || gpc->generation != slots->generation ||
kvm_is_error_hva(gpc->uhva)) {
gfn_t gfn = gpa_to_gfn(gpa);
gpc->gpa = gpa;
gpc->generation = slots->generation;
gpc->memslot = __gfn_to_memslot(slots, gfn);
gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
if (kvm_is_error_hva(gpc->uhva)) {
ret = -EFAULT;
goto out;
}
/*
* Even if the GPA and/or the memslot generation changed, the
* HVA may still be the same.
*/
if (gpc->uhva != old_uhva)
hva_change = true;
} else {
gpc->uhva = old_uhva;
}
}
/* Note: the offset must be correct before calling hva_to_pfn_retry() */
gpc->uhva += page_offset;
/*
* If the userspace HVA changed or the PFN was already invalid,
* drop the lock and do the HVA to PFN lookup again.
*/
if (!gpc->valid || hva_change) {
ret = hva_to_pfn_retry(gpc);
} else {
/*
* If the HVA→PFN mapping was already valid, don't unmap it.
* But do update gpc->khva because the offset within the page
* may have changed.
*/
gpc->khva = old_khva + page_offset;
ret = 0;
goto out_unlock;
}
out:
/*
* Invalidate the cache and purge the pfn/khva if the refresh failed.
* Some/all of the uhva, gpa, and memslot generation info may still be
* valid, leave it as is.
*/
if (ret) {
gpc->valid = false;
gpc->pfn = KVM_PFN_ERR_FAULT;
gpc->khva = NULL;
}
/* Detect a pfn change before dropping the lock! */
unmap_old = (old_pfn != gpc->pfn);
out_unlock:
write_unlock_irq(&gpc->lock);
if (unmap_old)
gpc_unmap(old_pfn, old_khva);
return ret;
}
int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len)
{
unsigned long uhva;
guard(mutex)(&gpc->refresh_lock);
if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len))
return -EINVAL;
/*
* If the GPA is valid then ignore the HVA, as a cache can be GPA-based
* or HVA-based, not both. For GPA-based caches, the HVA will be
* recomputed during refresh if necessary.
*/
uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva : KVM_HVA_ERR_BAD;
return __kvm_gpc_refresh(gpc, gpc->gpa, uhva);
}
void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
{
rwlock_init(&gpc->lock);
mutex_init(&gpc->refresh_lock);
gpc->kvm = kvm;
gpc->pfn = KVM_PFN_ERR_FAULT;
gpc->gpa = INVALID_GPA;
gpc->uhva = KVM_HVA_ERR_BAD;
gpc->active = gpc->valid = false;
}
static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
unsigned long len)
{
struct kvm *kvm = gpc->kvm;
if (!kvm_gpc_is_valid_len(gpa, uhva, len))
return -EINVAL;
guard(mutex)(&gpc->refresh_lock);
if (!gpc->active) {
if (KVM_BUG_ON(gpc->valid, kvm))
return -EIO;
spin_lock(&kvm->gpc_lock);
list_add(&gpc->list, &kvm->gpc_list);
spin_unlock(&kvm->gpc_lock);
/*
* Activate the cache after adding it to the list, a concurrent
* refresh must not establish a mapping until the cache is
* reachable by mmu_notifier events.
*/
write_lock_irq(&gpc->lock);
gpc->active = true;
write_unlock_irq(&gpc->lock);
}
return __kvm_gpc_refresh(gpc, gpa, uhva);
}
int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
{
/*
* Explicitly disallow INVALID_GPA so that the magic value can be used
* by KVM to differentiate between GPA-based and HVA-based caches.
*/
if (WARN_ON_ONCE(kvm_is_error_gpa(gpa)))
return -EINVAL;
return __kvm_gpc_activate(gpc, gpa, KVM_HVA_ERR_BAD, len);
}
int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva, unsigned long len)
{
return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len);
}
void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
{
struct kvm *kvm = gpc->kvm;
kvm_pfn_t old_pfn;
void *old_khva;
guard(mutex)(&gpc->refresh_lock);
if (gpc->active) {
/*
* Deactivate the cache before removing it from the list, KVM
* must stall mmu_notifier events until all users go away, i.e.
* until gpc->lock is dropped and refresh is guaranteed to fail.
*/
write_lock_irq(&gpc->lock);
gpc->active = false;
gpc->valid = false;
/*
* Leave the GPA => uHVA cache intact, it's protected by the
* memslot generation. The PFN lookup needs to be redone every
* time as mmu_notifier protection is lost when the cache is
* removed from the VM's gpc_list.
*/
old_khva = gpc->khva - offset_in_page(gpc->khva);
gpc->khva = NULL;
old_pfn = gpc->pfn;
gpc->pfn = KVM_PFN_ERR_FAULT;
write_unlock_irq(&gpc->lock);
spin_lock(&kvm->gpc_lock);
list_del(&gpc->list);
spin_unlock(&kvm->gpc_lock);
gpc_unmap(old_pfn, old_khva);
}
}