Re: [PATCH v2 2/2] x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()

[Jan Beulich <jbeulich@suse.com>]

On 31.07.2020 14:39, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> Re-factor the code to take advantage of the fact that the APIC access page is
> a 'special' page.

Hmm, that's going quite as far as I was thinking to go: In
particular, you leave in place the set_mmio_p2m_entry() use
in vmx_alloc_vlapic_mapping(). With that replaced, the
re-ordering in epte_get_entry_emt() that you do shouldn't
be necessary; you'd simple drop the checking of the
specific MFN. However, ...

> --- a/xen/arch/x86/hvm/mtrr.c
> +++ b/xen/arch/x86/hvm/mtrr.c
> @@ -814,29 +814,22 @@ int epte_get_entry_emt(struct domain *d, unsigned long gfn, mfn_t mfn,
>          return -1;
>      }
>  
> -    if ( direct_mmio )
> -    {
> -        if ( (mfn_x(mfn) ^ mfn_x(d->arch.hvm.vmx.apic_access_mfn)) >> order )
> -            return MTRR_TYPE_UNCACHABLE;
> -        if ( order )
> -            return -1;

... this part of the logic wants retaining, I think, i.e.
reporting back to the guest that the mapping needs splitting.
I'm afraid I have to withdraw my R-b on patch 1 for this
reason, as the check needs to be added there already.

Jan