Re: [PATCH] KVM: MTRR: fix fixed MTRR segment look up

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Alexis D...t" <alexis.dambricourt@gmail.com>
Cc: kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: MTRR: fix fixed MTRR segment look up
Date: Mon, 14 Dec 2015 16:36:27 +0100	[thread overview]
Message-ID: <566EE1FB.8050708@redhat.com> (raw)
In-Reply-To: <CAG0+HugZfEmVcBBZucgrHjBTJon7igm-i+=rHbTh0Wu+Ru-g_g@mail.gmail.com>

On 14/12/2015 15:39, Alexis D...t wrote:
> It fixes the slow-down of VM running with pci-passthrough, since some MTRR
> range changed from MTRR_TYPE_WRBACK to MTRR_TYPE_UNCACHABLE.
> 
> Fixes: fa61213746a ("KVM: MTRR: simplify kvm_mtrr_get_guest_memory_type")
> Bugzilla: (https://bugzilla.kernel.org/show_bug.cgi?id=107561)
> 
> Signed-off-by: Alexis Dambricourt <alexis.dambricourt@gmail.com>
> ---
>  arch/x86/kvm/mtrr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
> index 9e8bf13..adc54e1 100644
> --- a/arch/x86/kvm/mtrr.c
> +++ b/arch/x86/kvm/mtrr.c
> @@ -267,7 +267,7 @@ static int fixed_mtrr_addr_to_seg(u64 addr)
> 
>         for (seg = 0; seg < seg_num; seg++) {
>                 mtrr_seg = &fixed_seg_table[seg];
> -               if (mtrr_seg->start >= addr && addr < mtrr_seg->end)
> +               if (mtrr_seg->start <= addr && addr < mtrr_seg->end)

So this if could never be true.

>                         return seg;
>         }
> 
> --
> 

While that's embarrassing, :)  it would only apply to memory in the 640K-1M
range, while the logs in the BZ have

nov 22 17:06:49 Core-i7-5.lan kernel: vmx_get_mt_mask got the following: cpu=6, vcpu=0, gfn=67fe00, MMIO=0, cache=0

Note that this is a page above 4GB, which is why in the BZ I thought that
the culprit was MAXPHYADDR (and I still believe it is, at least for the
OVMF case---there are probably two bugs, and your patch fixes one).

Paolo