Re: [PATCH 3/3] KVM: MMU: consider host cache type in MMIO pfn check

[Ingo Molnar]

* Haozhong Zhang <haozhong.zhang@xxxxxxxxx> wrote:

> By default, KVM treats a reserved page as for MMIO purpose, and maps
> it to guest with UC memory type.  However, some reserved pages are not
> for MMIO, such as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> them with UC memory type will harm the performance.  In order to
> exclude those cases, we check the host cache mode in addition and only
> treat UC/UC- pages as MMIO.
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@xxxxxxxxx>
> Reported-by: Kumar, Karthik <karthik.kumar@xxxxxxxxx>
> ---
>  arch/x86/kvm/mmu.c | 32 +++++++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 0b481cc9c725..d4c821a6df3d 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2707,10 +2707,36 @@ 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));
> +
> +		/*
> +		 * By default, KVM treats a reserved page as for MMIO
> +		 * purpose, and maps it to guest with UC memory type.
> +		 * However, some reserved pages are not for MMIO, such
> +		 * as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> +		 * them with UC memory type will harm the performance.
> +		 * In order to exclude those cases, we check the host
> +		 * cache mode in addition and only treat UC/UC- pages
> +		 * as MMIO.
> +		 *
> +		 * track_pfn_insert() works only when PAT is enabled,
> +		 * so add pat_enabled() here.
> +		 */
> +		if (is_mmio && pat_enabled()) {
> +			pgprot_t prot;
> +			enum page_cache_mode cm;
> +
> +			track_pfn_insert(NULL, &prot, kvm_pfn_to_pfn(pfn));
> +			cm = pgprot2cachemode(prot);
> +			is_mmio = (cm == _PAGE_CACHE_MODE_UC ||
> +				   cm == _PAGE_CACHE_MODE_UC_MINUS);
> +		}
> +	}
> +
> +	return is_mmio;
>  }
>  
>  static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,

s/harm the performance
 /harm performance> 

But I suspect the rest of the comment should be rewritten too to be more fluid.

Beyond that - I think instead of exposing these low level details a properly named 
helper function should be put into pat.c instead - and KVM can use that.

Thanks,

	Ingo