linux-kernel - Re: [PATCH v4 01/12] KVM: x86/mmu: helpers to return if KVM honors guest MTRRs

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Date:   Mon, 9 Oct 2023 14:36:25 -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 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).

Oof, today is not my day.  I forgot to fix the missing "check" in the changelog
that Yan reported.  So *these* are the new hashes, barring yet another goof on
my end.

[1/5] KVM: x86/mmu: Add helpers to return if KVM honors guest MTRRs
      https://github.com/kvm-x86/linux/commit/1affe455d66d
[2/5] KVM: x86/mmu: Zap SPTEs when CR0.CD is toggled iff guest MTRRs are honored
      https://github.com/kvm-x86/linux/commit/7a18c7c2b69a
[3/5] KVM: x86/mmu: Zap SPTEs on MTRR update iff guest MTRRs are honored
      https://github.com/kvm-x86/linux/commit/9a3768191d95
[4/5] KVM: x86/mmu: Zap KVM TDP when noncoherent DMA assignment starts/stops
      https://github.com/kvm-x86/linux/commit/68c320298404
[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/8925b3194512