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Date: Mon, 9 Oct 2023 14:27:16 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Like Xu <like.xu.linux@...il.com>
Cc: Yan Zhao <yan.y.zhao@...el.com>, pbonzini@...hat.com,
chao.gao@...el.com, kai.huang@...el.com,
robert.hoo.linux@...il.com, yuan.yao@...ux.intel.com,
kvm list <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 01/12] KVM: x86/mmu: helpers to return if KVM honors
guest MTRRs
On Mon, Oct 09, 2023, Sean Christopherson wrote:
> On Sat, Oct 07, 2023, Like Xu wrote:
> > On 14/7/2023 2:50 pm, Yan Zhao wrote:
> > > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > > index 92d5a1924fc1..38bd449226f6 100644
> > > --- a/arch/x86/kvm/mmu.h
> > > +++ b/arch/x86/kvm/mmu.h
> > > @@ -235,6 +235,13 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> > > return -(u32)fault & errcode;
> > > }
> > > +bool __kvm_mmu_honors_guest_mtrrs(struct kvm *kvm, bool vm_has_noncoherent_dma);
> > > +
> > > +static inline bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm)
> > > +{
> > > + return __kvm_mmu_honors_guest_mtrrs(kvm, kvm_arch_has_noncoherent_dma(kvm));
> > > +}
> > > +
> > > void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end);
> > > int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 1e5db621241f..b4f89f015c37 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -4516,6 +4516,21 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
> > > }
> > > #endif
> > > +bool __kvm_mmu_honors_guest_mtrrs(struct kvm *kvm, bool vm_has_noncoherent_dma)
> >
> > According to the motivation provided in the comment, the function will no
> > longer need to be passed the parameter "struct kvm *kvm" but will rely on
> > the global parameters (plus vm_has_noncoherent_dma), removing "*kvm" ?
>
> Yeah, I'll fixup the commit to drop @kvm from the inner helper. Thanks!
Gah, and I gave more bad advice when I suggested this idea. There's no need to
explicitly check tdp_enabled, as shadow_memtype_mask is set to zero if TDP is
disabled. And that must be the case, e.g. make_spte() would generate a corrupt
shadow_memtype_mask were non-zero on Intel with shadow paging.
Yan, can you take a look at what I ended up with (see below) to make sure it
looks sane/acceptable to you?
New hashes (assuming I didn't botch things and need even more fixup).
[1/5] KVM: x86/mmu: Add helpers to return if KVM honors guest MTRRs
https://github.com/kvm-x86/linux/commit/ec1d8217d59b
[2/5] KVM: x86/mmu: Zap SPTEs when CR0.CD is toggled iff guest MTRRs are honored
https://github.com/kvm-x86/linux/commit/40de16c10b9d
[3/5] KVM: x86/mmu: Zap SPTEs on MTRR update iff guest MTRRs are honored
https://github.com/kvm-x86/linux/commit/defc3fae8d0f
[4/5] KVM: x86/mmu: Zap KVM TDP when noncoherent DMA assignment starts/stops
https://github.com/kvm-x86/linux/commit/b344d331adeb
[5/5] KVM: VMX: drop IPAT in memtype when CD=1 for KVM_X86_QUIRK_CD_NW_CLEARED
https://github.com/kvm-x86/linux/commit/a4d14445c47d
commit ec1d8217d59bd7cb03ae4e80551fee987be98a4e
Author: Yan Zhao <yan.y.zhao@...el.com>
Date: Fri Jul 14 14:50:06 2023 +0800
KVM: x86/mmu: Add helpers to return if KVM honors guest MTRRs
Add helpers to check if KVM honors guest MTRRs instead of open coding the
logic in kvm_tdp_page_fault(). Future fixes and cleanups will also need
to determine if KVM should honor guest MTRRs, e.g. for CR0.CD toggling and
and non-coherent DMA transitions.
Provide an inner helper, __kvm_mmu_honors_guest_mtrrs(), so that KVM can
if guest MTRRs were honored when stopping non-coherent DMA.
Note, there is no need to explicitly check that TDP is enabled, KVM clears
shadow_memtype_mask when TDP is disabled, i.e. it's non-zero if and only
if EPT is enabled.
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
Link: https://lore.kernel.org/r/20230714065006.20201-1-yan.y.zhao@intel.com
Link: https://lore.kernel.org/r/20230714065043.20258-1-yan.y.zhao@intel.com
[sean: squash into a one patch, drop explicit TDP check massage changelog]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 253fb2093d5d..bb8c86eefac0 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -237,6 +237,13 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
return -(u32)fault & errcode;
}
+bool __kvm_mmu_honors_guest_mtrrs(bool vm_has_noncoherent_dma);
+
+static inline bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm)
+{
+ return __kvm_mmu_honors_guest_mtrrs(kvm_arch_has_noncoherent_dma(kvm));
+}
+
void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end);
int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f7901cb4d2fa..5d3dc7119e57 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4479,21 +4479,28 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
}
#endif
+bool __kvm_mmu_honors_guest_mtrrs(bool vm_has_noncoherent_dma)
+{
+ /*
+ * If host MTRRs are ignored (shadow_memtype_mask is non-zero), and the
+ * VM has non-coherent DMA (DMA doesn't snoop CPU caches), KVM's ABI is
+ * to honor the memtype from the guest's MTRRs so that guest accesses
+ * to memory that is DMA'd aren't cached against the guest's wishes.
+ *
+ * Note, KVM may still ultimately ignore guest MTRRs for certain PFNs,
+ * e.g. KVM will force UC memtype for host MMIO.
+ */
+ return vm_has_noncoherent_dma && shadow_memtype_mask;
+}
+
int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
{
/*
* If the guest's MTRRs may be used to compute the "real" memtype,
* restrict the mapping level to ensure KVM uses a consistent memtype
- * across the entire mapping. If the host MTRRs are ignored by TDP
- * (shadow_memtype_mask is non-zero), and the VM has non-coherent DMA
- * (DMA doesn't snoop CPU caches), KVM's ABI is to honor the memtype
- * from the guest's MTRRs so that guest accesses to memory that is
- * DMA'd aren't cached against the guest's wishes.
- *
- * Note, KVM may still ultimately ignore guest MTRRs for certain PFNs,
- * e.g. KVM will force UC memtype for host MMIO.
+ * across the entire mapping.
*/
- if (shadow_memtype_mask && kvm_arch_has_noncoherent_dma(vcpu->kvm)) {
+ if (kvm_mmu_honors_guest_mtrrs(vcpu->kvm)) {
for ( ; fault->max_level > PG_LEVEL_4K; --fault->max_level) {
int page_num = KVM_PAGES_PER_HPAGE(fault->max_level);
gfn_t base = gfn_round_for_level(fault->gfn,
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