Re: [Xen-devel] [PATCH] x86/hvm: avoid p2m lookups for vlapic accesses.

[George Dunlap <dunlapg@xxxxxxxxx>]

On Thu, May 9, 2013 at 1:12 PM, Tim Deegan <tim@xxxxxxx> wrote:
> It seems like this patch helps the XP performance problem, and it
> doesn't hurt in other cases, so I think it should probably go in even if
> we find a general solution.
>
> commit ae8868ff4fc5a56e729399b031b27157d94febbf
> Author: Tim Deegan <tim@xxxxxxx>
> Date:   Thu May 9 13:06:53 2013 +0100
>
>     x86/hvm: avoid p2m lookups for vlapic accesses.
>
>     The LAPIC base address is a known GFN, so we can skip looking up the
>     p2m: we know it should be handled as emulated MMIO.  That helps
>     performance in older Windows OSes, which make a _lot_ of TPR accesses.
>
>     This will change the behaviour of any OS that maps other
>     memory/devices at its LAPIC address; the new behaviour (the LAPIC
>     mapping always wins) is closer to actual hardware behaviour.
>
>     Signed-off-by: Tim Deegan <tim@xxxxxxx>

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 7c3cb15..05ce054 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -1361,6 +1361,17 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
>          }
>      }
>
> +    /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
> +     * a fast path for LAPIC accesses, skipping the p2m lookup. */
> +    if ( !nestedhvm_vcpu_in_guestmode(v)
> +         && gfn == vlapic_base_address(vcpu_vlapic(v)) >> PAGE_SHIFT )
> +    {
> +        if ( !handle_mmio() )
> +            hvm_inject_hw_exception(TRAP_gp_fault, 0);
> +        rc = 1;
> +        goto out;
> +    }
> +
>      p2m = p2m_get_hostp2m(v->domain);
>      mfn = get_gfn_type_access(p2m, gfn, &p2mt, &p2ma,
>                                P2M_ALLOC | (access_w ? P2M_UNSHARE : 0), 
> NULL);
> @@ -2471,6 +2482,12 @@ static enum hvm_copy_result __hvm_copy(
>              gfn = addr >> PAGE_SHIFT;
>          }
>
> +        /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
> +         * a fast path for LAPIC accesses, skipping the p2m lookup. */
> +        if ( !nestedhvm_vcpu_in_guestmode(curr)
> +             && gfn == vlapic_base_address(vcpu_vlapic(curr)) >> PAGE_SHIFT )
> +            return HVMCOPY_bad_gfn_to_mfn;
> +
>          page = get_page_from_gfn(curr->domain, gfn, &p2mt, P2M_UNSHARE);
>
>          if ( p2m_is_paging(p2mt) )
>
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