linux-kernel - Re: [PATCH v3 08/11] KVM: x86: move vmx code to get EPT memtype when CR0.CD=1 to x86 common code

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Date:   Wed, 28 Jun 2023 15:57:09 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, chao.gao@...el.com, kai.huang@...el.com,
        robert.hoo.linux@...il.com
Subject: Re: [PATCH v3 08/11] KVM: x86: move vmx code to get EPT memtype when
 CR0.CD=1 to x86 common code

On Fri, Jun 16, 2023, Yan Zhao wrote:
> Move code in vmx.c to get cache disabled memtype when non-coherent DMA
> present to x86 common code.
> 
> This is the preparation patch for later implementation of fine-grained gfn
> zap for CR0.CD toggles when guest MTRRs are honored.
> 
> No functional change intended.
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  arch/x86/kvm/mtrr.c    | 19 +++++++++++++++++++
>  arch/x86/kvm/vmx/vmx.c | 10 +++++-----
>  arch/x86/kvm/x86.h     |  1 +
>  3 files changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
> index 3ce58734ad22..b35dd0bc9cad 100644
> --- a/arch/x86/kvm/mtrr.c
> +++ b/arch/x86/kvm/mtrr.c
> @@ -721,3 +721,22 @@ bool kvm_mtrr_check_gfn_range_consistency(struct kvm_vcpu *vcpu, gfn_t gfn,
>  
>  	return type == mtrr_default_type(mtrr_state);
>  }
> +
> +void kvm_mtrr_get_cd_memory_type(struct kvm_vcpu *vcpu, u8 *type, bool *ipat)

Hmm, I'm not convinced that this logic is subtle enough to warrant a common
helper with out params (I *really* don't like out params :-) ).

UC, or more specifically CR0.CD=1 on VMX without the quirk, is a super special case,
because to faithfully emulatee "No Fill" mode, KVM needs to ignore guest PAT (stupid WC).

I don't love having the same logic/assumptions in multiple places, but the CR0.CD=1
behavior is so rigidly tied to what KVM must do to that I think trying to provide a
common helper makes the code more complex than it needs to be.

If we open code the logic in the MTRR helper, than I think it can be distilled
down to:

	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
	bool mtrr_enabled = mtrr_is_enabled(mtrr_state);
	u8 default_type;

	/*
	 * Faithfully emulating CR0.CD=1 on VMX requires ignoring guest PAT, as
	 * WC in the PAT overrides UC in the MTRRs.  Zap all SPTEs so that KVM
	 * will once again start honoring guest PAT.
	 */
	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))
		return kvm_mtrr_zap_gfn_range(vcpu, gpa_to_gfn(0), gpa_to_gfn(~0ULL));

	default_type = mtrr_enabled ? mtrr_default_type(mtrr_state) :
				      mtrr_disabled_type(vcpu);

	if (default_type != MTRR_TYPE_WRBACK)
		return kvm_mtrr_zap_gfn_range(vcpu, gpa_to_gfn(0), gpa_to_gfn(~0ULL));

	if (mtrr_enabled) {
		if (gather_non_wb_fixed_mtrrs(vcpu, MTRR_TYPE_WRBACK))
			goto fail;

		if (gather_non_wb_var_mtrrs(vcpu, MTRR_TYPE_WRBACK))
			goto fail;

		kvm_zap_or_wait_mtrrs(vcpu->kvm);
	}

and this patch goes away.

> +{
> +	/*
> +	 * this routine is supposed to be called when guest mtrrs are honored
> +	 */
> +	if (unlikely(!kvm_mmu_honors_guest_mtrrs(vcpu->kvm))) {

I don't think this is worth checking, e.g. this would be WARN-worthy if it weren't
for an otherwise benign race with device (un)assignment.

> +		*type = MTRR_TYPE_WRBACK;
> +		*ipat = true;

> +	} else if (unlikely(!kvm_check_has_quirk(vcpu->kvm,

Eh, drop the "unlikely()" annotations.  AIUI, they almost never provide actual
performance benefits, and I dislike unnecessarily speculating on what userspace
is doing when it comes to code (though I 100% agree that this definitely unlikely)