Re: [Xen-devel] [PATCH (for 4.6)] x86/hvm: Unconditionally buffer writes to VRAM

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2015-07-16 at 10:34 +0100, Andrew Cooper wrote:
> On 16/07/15 10:24, Paul Durrant wrote:
> > When c/s 3bbaaec09 "unify stdvga mmio intercept with standard mmio
> > intercept" was added, a small semantic change was made. Prior to
> > this patch the hypervisor unconditionally sent all guest writes
> > to the VGA aperture as buffered ioreqs, whereas after the patch it
> > only does this when the VGA model is in 'stdvga' mode (sequencer
> > register #7 == 0).
> >
> > When installing Windows 7 (64-bit) using the default QEMU VGA model
> > (== cirrus), Windows leaves 'stdvga' mode early in boot and hence
> > all further writes to the VGA aperture are done using synchronous
> > ioreqs which slows down boot by several orders of magnitude (thanks
> > to the elaborate splash screen that Windows presents). This can be
> > viewed as a regression and so this patch re-instates previous
> > buffering behaviour.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Tested-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Keir Fraser <keir@xxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> This is unfortunate,

OOI why is it unfortunate? IOW why wouldn't we want buffer all accesses
to the VRAM (leaving aside that perhaps the original authors only
intended to do it for StdVGA).



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