Re: [PATCH v2 2/2] KVM: MMU: consider host cache mode in MMIO page check

[Xiao Guangrong <xiaoguangrong.eric@gmail.com>]

On 10/31/2017 07:48 PM, 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 by lookup_memtype() in
> addition and only treat UC/UC- pages as MMIO.
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@intel.com>
> Reported-by: Kumar, Karthik <karthik.kumar@intel.com>
> ---
>   arch/x86/kvm/mmu.c | 30 +++++++++++++++++++++++++++---
>   1 file changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 0b481cc9c725..206828d18857 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2707,10 +2707,34 @@ 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));
> +	bool is_mmio = true;
>   
> -	return true;
> +	if (pfn_valid(pfn)) {
> +		is_mmio = !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 by lookup_memtype() in addition and
> +		 * only treat UC/UC- pages as MMIO.
> +		 *
> +		 * lookup_memtype() works only when PAT is enabled, so
> +		 * add pat_enabled() check here.
> +		 */
> +		if (is_mmio && pat_enabled()) {
> +			enum page_cache_mode cm = lookup_memtype(PFN_PHYS(pfn));
> +
> +			is_mmio = (cm == _PAGE_CACHE_MODE_UC ||
> +				   cm == _PAGE_CACHE_MODE_UC_MINUS);
> +		}
> +	}

You can move all of these detailed stuffs to pat.c and abstract them by introducing
a function, maybe named pat_pfn_is_uc(). I think this is what Ingo wants.