Date: Wed, 23 May 2012 20:37:39 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Avi Kivity <avi@redhat.com>
cc: "Michael S. Tsirkin" <mst@redhat.com>, kvm@vger.kernel.org,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kvm: optimize ISR lookups
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References: <20120521163727.GA13337@redhat.com> <alpine.LFD.2.02.1205211940030.3231@ionos> <4FBB7185.6040105@redhat.com> <alpine.LFD.2.02.1205221919260.3231@ionos> <4FBCFDD1.5050405@redhat.com>
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On Wed, 23 May 2012, Avi Kivity wrote:

> On 05/22/2012 08:26 PM, Thomas Gleixner wrote:
> > On Tue, 22 May 2012, Avi Kivity wrote:
> >> On 05/22/2012 12:04 AM, Thomas Gleixner wrote:
> >> > The only justification for having the same layout as the actual
> >> > hardware is when you are going to map the memory into the guest space,
> >> > which is not the case here.
> >>
> >> The APIC page is in fact mapped  to the hardware (not the guest, but vmx
> >> microcode does access it).  Only one register, the TPR, is ever used. 
> >> It's possible to re-layout the data structure so that the TPR stays in
> >> the same place while everything else becomes contiguous, but we'll have
> >> to do it again if the hardware starts mapping more registers.
> > 
> > I would avoid that by having a compressed version which reflects the
> > SW state and the mapped one which allows the vmx microcode to fiddle
> > with the TPR. If you need more registers in the HW page then you don't
> > have to worry about the layout and just have a proper accessor for
> > that.
> 
> That works, but replaces one problem with another: now we have two
> sources for the same data, and need to juggle between them depending on
> register number (either synchronizing in both directions, or special
> casing); so you're simplifying one thing at the expense of the other.
> If the microcode starts accessing more registers, then having two
> layouts becomes even uglier.

Fair enough :)
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