linux-kernel - Re: [PATCH v5 2/2] KVM: MMU: consider host cache mode in MMIO page check

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Date:   Fri, 15 Dec 2017 10:14:24 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Haozhong Zhang <haozhong.zhang@...el.com>, kvm@...r.kernel.org,
        x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, rkrcmar@...hat.com,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        ivan.d.cuevas.escareno@...el.com, karthik.kumar@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Olif Chapman <olif.chapman@...cle.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v5 2/2] KVM: MMU: consider host cache mode in MMIO page
 check

On 08/11/2017 08:56, Haozhong Zhang wrote:
> Some reserved pages, such as those from NVDIMM DAX devices, are not
> for MMIO, and can be mapped with cached memory type for better
> performance. However, the above check misconceives those pages as
> MMIO.  Because KVM maps MMIO pages with UC memory type, the
> performance of guest accesses to those pages would be harmed.
> Therefore, we check the host memory type in addition and only treat
> UC/UC- pages as MMIO.
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com>
> Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@...el.com>
> Reported-by: Kumar, Karthik <karthik.kumar@...el.com>

WC should be allowed as well, because the combination of EPT_PAT=UC and
gPAT=WC gives WC effective memory type.

Maybe it's better after all if the lookup_memtype call remains in
kvm_is_mmio_pfn, like

	if (!pfn_valid(pfn))
		return true;

	if (is_zero_pfn(pfn) || !PageReserved(pfn_to_page(pfn))
		return false;

	/* ... long comment ... */
	if (!pat_enabled()
		return true;
	else {
		cm = lookup_memtype(PFN_PHYS(pfn));
		return cm == ...
	}

or something like that.

Thanks,

Paolo

> ---
>  arch/x86/kvm/mmu.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 0b481cc9c725..7715476bc5c9 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2708,7 +2708,20 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
>  static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
>  {
>  	if (pfn_valid(pfn))
> -		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
> +		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
> +			/*
> +			 * Some reserved pages, such as those from
> +			 * NVDIMM DAX devices, are not for MMIO, and
> +			 * can be mapped with cached memory type for
> +			 * better performance. However, the above
> +			 * check misconceives those pages as MMIO.
> +			 * Because KVM maps MMIO pages with UC memory
> +			 * type, the performance of guest accesses to
> +			 * those pages would be harmed. Therefore, we
> +			 * check the host memory type in addition and
> +			 * only treat UC/UC- pages as MMIO.
> +			 */
> +			(!pat_enabled() || pat_pfn_is_uc_or_uc_minus(pfn));
>  
>  	return true;
>  }
>