[RFC PATCH 1/3] KVM: vmx: rewrite the comment in vmx_get_mt_mask

[Paolo Bonzini]

On 13/02/20 22:30, Chia-I Wu wrote:
> Better reflect the structure of the code and metion why we could not
> always honor the guest.
> 
> Signed-off-by: Chia-I Wu <olvaffe at gmail.com>
> Cc: Gurchetan Singh <gurchetansingh at chromium.org>
> Cc: Gerd Hoffmann <kraxel at redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3be25ecae145..266ef87042da 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6854,17 +6854,24 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
>  	u8 cache;
>  	u64 ipat = 0;
>  
> -	/* For VT-d and EPT combination
> -	 * 1. MMIO: always map as UC
> -	 * 2. EPT with VT-d:
> -	 *   a. VT-d without snooping control feature: can't guarantee the
> -	 *	result, try to trust guest.
> -	 *   b. VT-d with snooping control feature: snooping control feature of
> -	 *	VT-d engine can guarantee the cache correctness. Just set it
> -	 *	to WB to keep consistent with host. So the same as item 3.
> -	 * 3. EPT without VT-d: always map as WB and set IPAT=1 to keep
> -	 *    consistent with host MTRR
> +	/* We wanted to honor guest CD/MTRR/PAT, but doing so could result in
> +	 * memory aliases with conflicting memory types and sometimes MCEs.
> +	 * We have to be careful as to what are honored and when.
> +	 *
> +	 * For MMIO, guest CD/MTRR are ignored.  The EPT memory type is set to
> +	 * UC.  The effective memory type is UC or WC depending on guest PAT.
> +	 * This was historically the source of MCEs and we want to be
> +	 * conservative.
> +	 *
> +	 * When there is no need to deal with noncoherent DMA (e.g., no VT-d
> +	 * or VT-d has snoop control), guest CD/MTRR/PAT are all ignored.  The
> +	 * EPT memory type is set to WB.  The effective memory type is forced
> +	 * WB.
> +	 *
> +	 * Otherwise, we trust guest.  Guest CD/MTRR/PAT are all honored.  The
> +	 * EPT memory type is used to emulate guest CD/MTRR.
>  	 */
> +
>  	if (is_mmio) {
>  		cache = MTRR_TYPE_UNCACHABLE;
>  		goto exit;
> 

This is certainly an improvement, especially the part that points out
how guest PAT still allows MMIO to be handled as WC.

Thanks,

Paolo