Re: [PATCH v1 2/3] KVM: x86: Handle the case of 5-level shadow page table
From: Wei Huang
Date: Sun Aug 08 2021 - 13:49:46 EST
On 8/6/21 12:58 PM, Sean Christopherson wrote:
On Thu, Aug 05, 2021, Wei Huang wrote:
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 44e4561e41f5..b162c3e530aa 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3428,7 +3428,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
* the shadow page table may be a PAE or a long mode page table.
*/
pm_mask = PT_PRESENT_MASK | shadow_me_mask;
- if (mmu->shadow_root_level == PT64_ROOT_4LEVEL) {
+ if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
pm_mask |= PT_ACCESSED_MASK | PT_WRITABLE_MASK | PT_USER_MASK;
if (WARN_ON_ONCE(!mmu->pml4_root)) {
@@ -3454,11 +3454,17 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
PT32_ROOT_LEVEL, false);
mmu->pae_root[i] = root | pm_mask;
}
+ mmu->root_hpa = __pa(mmu->pae_root);
- if (mmu->shadow_root_level == PT64_ROOT_4LEVEL)
+ if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
+ mmu->pml4_root[0] = mmu->root_hpa | pm_mask;
mmu->root_hpa = __pa(mmu->pml4_root);
- else
- mmu->root_hpa = __pa(mmu->pae_root);
+ }
+
+ if (mmu->shadow_root_level == PT64_ROOT_5LEVEL) {
+ mmu->pml5_root[0] = mmu->root_hpa | pm_mask;
+ mmu->root_hpa = __pa(mmu->pml5_root);
+ }
Ouch, the root_hpa chaining is subtle. That's my fault :-) I think it would be
better to explicitly chain pae->pml4->pml5? E.g.
if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
mmu->pml4_root[0] = __pa(mmu->pae_root) | pm_mask;
if (mmu->shadow_root_level == PT64_ROOT_5LEVEL) {
mmu->pml5_root[0] = __pa(mmu->pml4_root) | pm_mask;
mmu->root_hpa = __pa(mmu->pml5_root);
} else {
mmu->root_hpa = __pa(mmu->pml4_root);
}
} else {
mmu->root_hpa = __pa(mmu->pae_root);
}
It'd require more churn if we get to 6-level paging, but that's a risk I'm willing
to take ;-)
Thanks for the review. This part of code is indeed subtle. The chaining
trick will be easier to understand with a proper explanation. My
proposal is to keep the original approach, but add more comments to this
group of code.
/*
* Depending on the shadow_root_level, build the root_hpa table
by
* chaining either pml5->pml4->pae or pml4->pae.
*/
mmu->root_hpa = __pa(mmu->pae_root);
if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
mmu->pml4_root[0] = mmu->root_hpa | pm_mask;
mmu->root_hpa = __pa(mmu->pml4_root);
}
if (mmu->shadow_root_level == PT64_ROOT_5LEVEL) {
mmu->pml5_root[0] = mmu->root_hpa | pm_mask;
mmu->root_hpa = __pa(mmu->pml5_root);
}
This code will be easy to extend for 6-level page table (if needed) in
the future.
set_root_pgd:
mmu->root_pgd = root_pgd;
@@ -3471,7 +3477,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
{
struct kvm_mmu *mmu = vcpu->arch.mmu;
- u64 *pml4_root, *pae_root;
+ u64 *pml5_root, *pml4_root, *pae_root;
/*
* When shadowing 32-bit or PAE NPT with 64-bit NPT, the PML4 and PDP
@@ -3487,17 +3493,18 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
* This mess only works with 4-level paging and needs to be updated to
* work with 5-level paging.
*/
- if (WARN_ON_ONCE(mmu->shadow_root_level != PT64_ROOT_4LEVEL))
+ if (WARN_ON_ONCE(mmu->shadow_root_level < PT64_ROOT_4LEVEL)) {
This is amusingly wrong. The check above this is:
if (mmu->direct_map || mmu->root_level >= PT64_ROOT_4LEVEL ||
mmu->shadow_root_level < PT64_ROOT_4LEVEL) <--------
return 0;
meaning this is dead code. It should simply deleted. If we reaaaaaly wanted to
future proof the code, we could do:
if (WARN_ON_ONCE(mmu->shadow_root_level > PT64_ROOT_5LEVEL)
return -EIO;
but at that point we're looking at a completely different architecture, so I don't
think we need to be that paranoid :-)
You are right that this can be removed.
return -EIO;
+ }
- if (mmu->pae_root && mmu->pml4_root)
+ if (mmu->pae_root && mmu->pml4_root && mmu->pml5_root)
return 0;
/*
* The special roots should always be allocated in concert. Yell and
* bail if KVM ends up in a state where only one of the roots is valid.
*/
- if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root))
+ if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root || mmu->pml5_root))
return -EIO;
/*
@@ -3506,18 +3513,30 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
*/
pae_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
if (!pae_root)
- return -ENOMEM;
+ goto err_out;
Branching to the error handling here is silly, it's the first allocation.
pml4_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
- if (!pml4_root) {
- free_page((unsigned long)pae_root);
- return -ENOMEM;
- }
+ if (!pml4_root)
+ goto err_out;
+
+ pml5_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
This should be guarded by "mmu->shadow_root_level > PT64_ROOT_4LEVEL", there's no
need to waste a page on PML5 if it can't exist.
Will do
+ if (!pml5_root)
+ goto err_out;
mmu->pae_root = pae_root;
mmu->pml4_root = pml4_root;
+ mmu->pml5_root = pml5_root;
return 0;
+err_out:
+ if (pae_root)
+ free_page((unsigned long)pae_root);
+ if (pml4_root)
+ free_page((unsigned long)pml4_root);
+ if (pml5_root)
+ free_page((unsigned long)pml5_root);
This is flawed as failure to allocate pml4_root will consume an uninitialized
pml5_root. There's also no need to check for non-NULL values as free_page plays
nice with NULL pointers.
If you drop the unnecessary goto for pae_root allocation failure, than this can
become:
err_out:
free_page((unsigned long)pml4_root);
free_page((unsigned long)pae_root);
since pml4_root will be NULL if pml4_root allocation failures. IMO that's
unnecessarily clever though, and a more standard:
err_pml5:
free_page((unsigned long)pml4_root);
err_pml4:
free_page((unsigned long)pae_root);
return -ENOMEM;
would be far easier to read/maintain.
I will take the advice for this part of code.
+
+ return -ENOMEM;
}
void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
@@ -5320,6 +5339,7 @@ static void free_mmu_pages(struct kvm_mmu *mmu)
set_memory_encrypted((unsigned long)mmu->pae_root, 1);
free_page((unsigned long)mmu->pae_root);
free_page((unsigned long)mmu->pml4_root);
+ free_page((unsigned long)mmu->pml5_root);
}
static int __kvm_mmu_create(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
--
2.31.1