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Message-ID: <ZoZYryszKWI5IYy5@yzhao56-desk.sh.intel.com>
Date: Thu, 4 Jul 2024 16:09:19 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>,
"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...il.com"
<isaku.yamahata@...il.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
"dmatlack@...gle.com" <dmatlack@...gle.com>, "kvm@...r.kernel.org"
<kvm@...r.kernel.org>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v3 13/17] KVM: x86/tdp_mmu: Support mirror root for TDP
MMU
On Thu, Jul 04, 2024 at 03:40:35AM +0800, Edgecombe, Rick P wrote:
> On Wed, 2024-06-26 at 13:05 +0800, Yan Zhao wrote:
> > But we still need the reload path to have each vcpu to hold a ref count of the
> > mirror root.
> > What about below fix?
> >
> > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > index 026e8edfb0bd..4decd13457ec 100644
> > --- a/arch/x86/kvm/mmu.h
> > +++ b/arch/x86/kvm/mmu.h
> > @@ -127,9 +127,28 @@ void kvm_mmu_sync_prev_roots(struct kvm_vcpu *vcpu);
> > void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
> > int bytes);
> >
> > +static inline bool kvm_has_mirrored_tdp(const struct kvm *kvm)
> > +{
> > + return kvm->arch.vm_type == KVM_X86_TDX_VM;
> > +}
> > +
> > +static inline bool kvm_mmu_root_hpa_is_invalid(struct kvm_vcpu *vcpu)
> > +{
> > + return vcpu->arch.mmu->root.hpa == INVALID_PAGE;
> > +}
> > +
> > +static inline bool kvm_mmu_mirror_root_hpa_is_invalid(struct kvm_vcpu *vcpu)
> > +{
> > + if (!kvm_has_mirrored_tdp(vcpu->kvm))
> > + return false;
> > +
> > + return vcpu->arch.mmu->mirror_root_hpa == INVALID_PAGE;
> > +}
> > +
> > static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
> > {
> > - if (likely(vcpu->arch.mmu->root.hpa != INVALID_PAGE))
> > + if (!kvm_mmu_root_hpa_is_invalid(vcpu) &&
> > + !kvm_mmu_mirror_root_hpa_is_invalid(vcpu))
>
> If we go this way, we should keep the likely. But, I'm not convinced.
>
> > return 0;
> >
> > return kvm_mmu_load(vcpu);
> > @@ -322,11 +341,6 @@ static inline gpa_t kvm_translate_gpa(struct kvm_vcpu
> > *vcpu,
> > return translate_nested_gpa(vcpu, gpa, access, exception);
> > }
> >
> > -static inline bool kvm_has_mirrored_tdp(const struct kvm *kvm)
> > -{
> > - return kvm->arch.vm_type == KVM_X86_TDX_VM;
> > -}
> > -
> > static inline gfn_t kvm_gfn_direct_bits(const struct kvm *kvm)
> > {
> > return kvm->arch.gfn_direct_bits;
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index e1299eb03e63..5e7d92074f70 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3702,9 +3702,11 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu
> > *vcpu)
> > int r;
> >
> > if (tdp_mmu_enabled) {
> > - if (kvm_has_mirrored_tdp(vcpu->kvm))
> > + if (kvm_mmu_mirror_root_hpa_is_invalid(vcpu))
> > kvm_tdp_mmu_alloc_root(vcpu, true);
> > - kvm_tdp_mmu_alloc_root(vcpu, false);
> > +
> > + if (kvm_mmu_root_hpa_is_invalid(vcpu))
> > + kvm_tdp_mmu_alloc_root(vcpu, false);
>
> So we can have either:
> mirror_root_hpa = INVALID_PAGE
> root.hpa = INVALID_PAGE
>
> or:
> mirror_root_hpa = root
> root.hpa = INVALID_PAGE
>
> We don't ever have:
> mirror_root_hpa = INVALID_PAGE
> root.hpa = root
>
> Because mirror_root_hpa is special.
>
Good point.
> > return 0;
> > }
> >
>
> Having the check in kvm_mmu_reload() and here both is really ugly. Right now we
> have kvm_mmu_reload() which only allocates new roots if needed, then
> kvm_mmu_load() goes and allocates/references them. Now mmu_alloc_direct_roots()
> has the same logic to only reload as needed.
>
> So could we just leave the kvm_mmu_reload() logic as is, and just have special
> logic in mmu_alloc_direct_roots() to not avoid re-allocating mirror roots? This
> is actually what v19 had, but it was thought to be a historical relic:
> "Historically we needed it. We don't need it now. We can drop it."
> https://lore.kernel.org/kvm/20240325200122.GH2357401@ls.amr.corp.intel.com/
>
> So just have this small fixup instead?
Yes.
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index ae5d5dee86af..2e062178222e 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3705,7 +3705,8 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
> int r;
>
> if (tdp_mmu_enabled) {
> - if (kvm_has_mirrored_tdp(vcpu->kvm))
> + if (kvm_has_mirrored_tdp(vcpu->kvm) &&
> + !VALID_PAGE(mmu->mirror_root_hpa))
> kvm_tdp_mmu_alloc_root(vcpu, true);
> kvm_tdp_mmu_alloc_root(vcpu, false);
> return 0;
>
Perhaps also a comment in kvm_mmu_reload() to address concerns like why checking
only root.hpa in kvm_mmu_reload() is enough.
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 03737f3aaeeb..aba98c8cc67d 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -129,6 +129,15 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
{
+ /*
+ * Checking root.hpa is sufficient even when KVM has mirror root.
+ * We can have either:
+ * (1) mirror_root_hpa = INVALID_PAGE, root.hpa = INVALID_PAGE
+ * (2) mirror_root_hpa = root , root.hpa = INVALID_PAGE
+ * (3) mirror_root_hpa = root1, root.hpa = root2
+ * We don't ever have:
+ * mirror_root_hpa = INVALID_PAGE, root.hpa = root
+ */
if (likely(vcpu->arch.mmu->root.hpa != INVALID_PAGE))
return 0;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a5f803f1d17e..eee35e958971 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3705,7 +3705,8 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
int r;
if (tdp_mmu_enabled) {
- if (kvm_has_mirrored_tdp(vcpu->kvm))
+ if (kvm_has_mirrored_tdp(vcpu->kvm) &&
+ !VALID_PAGE(mmu->mirror_root_hpa))
kvm_tdp_mmu_alloc_root(vcpu, true);
kvm_tdp_mmu_alloc_root(vcpu, false);
return 0;
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