Re: [PATCH v4 7/9] KVM: x86: When guest set CR3, handle LAM bits semantics

Chao Gao
2023-02-13T03:31:32Z

On Thu, Feb 09, 2023 at 10:40:20AM +0800, Robert Hoo wrote:
>When only changes LAM bits, ask next vcpu run to load mmu pgd, so that it
>will build new CR3 with LAM bits updates.
>When changes on CR3's effective address bits, no matter LAM bits changes or not,
>go through normal pgd update process.

Please squash this into patch 2.

>
>Reviewed-by: Jingqi Liu <jingqi.liu@intel.com>
>Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
>---
> arch/x86/kvm/x86.c | 24 ++++++++++++++++++++----
> 1 file changed, 20 insertions(+), 4 deletions(-)
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 3218f465ae71..7df6c9dd12a5 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -1242,9 +1242,9 @@ static bool kvm_is_valid_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> {
> bool skip_tlb_flush = false;
>- unsigned long pcid = 0;
>+ unsigned long pcid = 0, old_cr3;
> #ifdef CONFIG_X86_64

>- bool pcid_enabled = kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE);
>+ bool pcid_enabled = !!kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE);

This change isn't related. Please drop it or post it separately.

>
> if (pcid_enabled) {
> skip_tlb_flush = cr3 & X86_CR3_PCID_NOFLUSH;
>@@ -1257,6 +1257,10 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> if (cr3 == kvm_read_cr3(vcpu) && !is_pae_paging(vcpu))
> goto handle_tlb_flush;
>
>+ if (!guest_cpuid_has(vcpu, X86_FEATURE_LAM) &&
>+ (cr3 & (X86_CR3_LAM_U48 | X86_CR3_LAM_U57)))
>+ return 1;

can you move this check to kvm_vcpu_is_valid_cr3(), i.e., return false in
that function if any LAM bit is toggled while LAM isn't exposed to the guest?

>+
> /*
> * Do not condition the GPA check on long mode, this helper is used to
> * stuff CR3, e.g. for RSM emulation, and there is no guarantee that
>@@ -1268,8 +1272,20 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> if (is_pae_paging(vcpu) && !load_pdptrs(vcpu, cr3))
> return 1;
>
>- if (cr3 != kvm_read_cr3(vcpu))
>- kvm_mmu_new_pgd(vcpu, cr3);
>+ old_cr3 = kvm_read_cr3(vcpu);
>+ if (cr3 != old_cr3) {
>+ if ((cr3 ^ old_cr3) & CR3_ADDR_MASK) {

Does this check against CR3_ADDR_MASK necessarily mean LAM bits are
toggled, i.e., CR3_ADDR_MASK == ~(X86_CR3_LAM_U48 | X86_CR3_LAM_U57)?

Why not check if LAM bits are changed? This way the patch only changes
cases related to LAM, keeping other cases intact.

>+ kvm_mmu_new_pgd(vcpu, cr3 & ~(X86_CR3_LAM_U48 |
>+ X86_CR3_LAM_U57));

Do you need to touch kvm_mmu_new_pgd() in nested_vmx_load_cr3()?

>+ } else {
>+ /*
>+ * Though effective addr no change, mark the
>+ * request so that LAM bits will take effect
>+ * when enter guest.
>+ */
>+ kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
>+ }
>+ }
>
> vcpu->arch.cr3 = cr3;
> kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
>--
>2.31.1
>