linux-kernel - Re: [PATCH] KVM: SVM: obey guest PAT

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Message-ID: <20171110212718.GA2189@flask>
Date:   Fri, 10 Nov 2017 22:27:19 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: obey guest PAT

2017-10-26 09:13+0200, Paolo Bonzini:
> For many years some users of assigned devices have reported worse
> performance on AMD processors with NPT than on AMD without NPT,
> Intel or bare metal.
> 
> The reason turned out to be that SVM is discarding the guest PAT
> setting and uses the default (PA0=PA4=WB, PA1=PA5=WT, PA2=PA6=UC-,
> PA3=UC).  The guest might be using a different setting, and
> especially might want write combining but isn't getting it
> (instead getting slow UC or UC- accesses).
> 
> Thanks a lot to geoff@...tfission.com for noticing the relation
> to the g_pat setting.  The patch has been tested also by a bunch
> of people on VFIO users forums.
> 
> Fixes: 709ddebf81cb40e3c36c6109a7892e8b93a09464
> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=196409
> Cc: stable@...r.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Applied, thanks.