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KVM: x86/mmu: look for a cached PGD when going from 32-bit to 64-bit 

Right now, PGD caching avoids placing a PAE root in the cache by using the
old value of mmu->root_level and mmu->shadow_root_level; it does not look
for a cached PGD if the old root is a PAE one, and then frees it using
kvm_mmu_free_roots.

Change the logic instead to free the uncacheable root early.
This way, __kvm_new_mmu_pgd is able to look up the cache when going from
32-bit to 64-bit (if there is a hit, the invalid root becomes the least
recently used).  An example of this is nested virtualization with shadow
paging, when a 64-bit L1 runs a 32-bit L2.

As a side effect (which is actually the reason why this patch was
written), PGD caching does not use the old value of mmu->root_level
and mmu->shadow_root_level anymore.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-02-09 02:49:47 -05:00
parent 0c1c92f15f
commit 5499ea73e7
1 changed files with 59 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

82
arch/x86/kvm/mmu/mmu.c
View File

@ -4085,52 +4085,88 @@ static inline bool is_root_usable(struct kvm_mmu_root_info *root, gpa_t pgd,
union kvm_mmu_page_role role)
{
return (role.direct || pgd == root->pgd) &&
VALID_PAGE(root->hpa) && to_shadow_page(root->hpa) &&
VALID_PAGE(root->hpa) &&
role.word == to_shadow_page(root->hpa)->role.word;
}
/*
* Find out if a previously cached root matching the new pgd/role is available.
* The current root is also inserted into the cache.
* If a matching root was found, it is assigned to kvm_mmu->root.hpa and true is
* returned.
* Otherwise, the LRU root from the cache is assigned to kvm_mmu->root.hpa and
* false is returned. This root should now be freed by the caller.
* Find out if a previously cached root matching the new pgd/role is available,
* and insert the current root as the MRU in the cache.
* If a matching root is found, it is assigned to kvm_mmu->root and
* true is returned.
* If no match is found, kvm_mmu->root is left invalid, the LRU root is
* evicted to make room for the current root, and false is returned.
*/
static bool cached_root_available(struct kvm_vcpu *vcpu, gpa_t new_pgd,
union kvm_mmu_page_role new_role)
static bool cached_root_find_and_keep_current(struct kvm *kvm, struct kvm_mmu *mmu,
gpa_t new_pgd,
union kvm_mmu_page_role new_role)
{
uint i;
struct kvm_mmu *mmu = vcpu->arch.mmu;
if (is_root_usable(&mmu->root, new_pgd, new_role))
return true;
for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++) {
/*
* The swaps end up rotating the cache like this:
* C 0 1 2 3 (on entry to the function)
* 0 C 1 2 3
* 1 C 0 2 3
* 2 C 0 1 3
* 3 C 0 1 2 (on exit from the loop)
*/
swap(mmu->root, mmu->prev_roots[i]);
if (is_root_usable(&mmu->root, new_pgd, new_role))
break;
return true;
}
return i < KVM_MMU_NUM_PREV_ROOTS;
kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
return false;
}
static bool fast_pgd_switch(struct kvm_vcpu *vcpu, gpa_t new_pgd,
union kvm_mmu_page_role new_role)
/*
* Find out if a previously cached root matching the new pgd/role is available.
* On entry, mmu->root is invalid.
* If a matching root is found, it is assigned to kvm_mmu->root, the LRU entry
* of the cache becomes invalid, and true is returned.
* If no match is found, kvm_mmu->root is left invalid and false is returned.
*/
static bool cached_root_find_without_current(struct kvm *kvm, struct kvm_mmu *mmu,
gpa_t new_pgd,
union kvm_mmu_page_role new_role)
{
struct kvm_mmu *mmu = vcpu->arch.mmu;
uint i;
for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
if (is_root_usable(&mmu->prev_roots[i], new_pgd, new_role))
goto hit;
return false;
hit:
swap(mmu->root, mmu->prev_roots[i]);
/* Bubble up the remaining roots. */
for (; i < KVM_MMU_NUM_PREV_ROOTS - 1; i++)
mmu->prev_roots[i] = mmu->prev_roots[i + 1];
mmu->prev_roots[i].hpa = INVALID_PAGE;
return true;
}
static bool fast_pgd_switch(struct kvm *kvm, struct kvm_mmu *mmu,
gpa_t new_pgd, union kvm_mmu_page_role new_role)
{
/*
* For now, limit the fast switch to 64-bit hosts+VMs in order to avoid
* For now, limit the caching to 64-bit hosts+VMs in order to avoid
* having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
* later if necessary.
*/
if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
mmu->root_level >= PT64_ROOT_4LEVEL)
return cached_root_available(vcpu, new_pgd, new_role);
if (VALID_PAGE(mmu->root.hpa) && !to_shadow_page(mmu->root.hpa))
kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
return false;
if (VALID_PAGE(mmu->root.hpa))
return cached_root_find_and_keep_current(kvm, mmu, new_pgd, new_role);
else
return cached_root_find_without_current(kvm, mmu, new_pgd, new_role);
}
static void __kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd,
@ -4138,8 +4174,8 @@ static void __kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd,
{
struct kvm_mmu *mmu = vcpu->arch.mmu;
if (!fast_pgd_switch(vcpu, new_pgd, new_role)) {
kvm_mmu_free_roots(vcpu->kvm, mmu, KVM_MMU_ROOT_CURRENT);
if (!fast_pgd_switch(vcpu->kvm, mmu, new_pgd, new_role)) {
/* kvm_mmu_ensure_valid_pgd will set up a new root. */
return;
}