Re: [PATCH 3/3] x86/ept: force WB cache attributes for grant and foreign maps

[Jan Beulich <jbeulich@suse.com>]

On 28.05.2021 19:39, Roger Pau Monne wrote:
> --- a/xen/arch/x86/mm/p2m-ept.c
> +++ b/xen/arch/x86/mm/p2m-ept.c
> @@ -487,11 +487,12 @@ static int ept_invalidate_emt_range(struct p2m_domain *p2m,
>  }
>  
>  int epte_get_entry_emt(struct domain *d, gfn_t gfn, mfn_t mfn,
> -                       unsigned int order, bool *ipat, bool direct_mmio)
> +                       unsigned int order, bool *ipat, p2m_type_t type)
>  {
>      int gmtrr_mtype, hmtrr_mtype;
>      struct vcpu *v = current;
>      unsigned long i;
> +    bool direct_mmio = type == p2m_mmio_direct;

I don't think this variable is worthwhile to retain/introduce:

> @@ -535,9 +536,33 @@ int epte_get_entry_emt(struct domain *d, gfn_t gfn, mfn_t mfn,
>          }
>      }
>  
> -    if ( direct_mmio )

With this gone, there's exactly one further use left. Preferably
with this adjustment (which I'd be fine to make while committing, as
long as you and/or the maintainers agree)
Reviewed-by: Jan Beulich <jbeulich@suse.com>

> +    switch ( type )
> +    {
> +    case p2m_mmio_direct:
>          return MTRR_TYPE_UNCACHABLE;

As a largely unrelated note: We really want to find a way to return
WC here for e.g. the frame buffer of graphics cards, the more that
hvm_get_mem_pinned_cacheattr() gets invoked only below from here
(unlike at initial introduction of the function, where it was called
ahead of the direct_mmio check, but still after the mfn_valid(), so
the results were inconsistent anyway). Perhaps we should obtain the
host MTRR setting for the page (or range) in question.

As to hvm_get_mem_pinned_cacheattr(), XEN_DOMCTL_pin_mem_cacheattr
is documented to be intended to be used on RAM only anyway ...

Jan