Re: [Xen-devel] OVMF very slow on AMD

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Jul 28, 2016 at 11:54:27AM +0100, Andrew Cooper wrote:
> On 28/07/16 11:43, George Dunlap wrote:
> > On Thu, Jul 28, 2016 at 11:18 AM, Anthony PERARD
> > <anthony.perard@xxxxxxxxxx> wrote:
> >> On Wed, Jul 27, 2016 at 03:45:23PM -0400, Boris Ostrovsky wrote:
> >>> On 07/27/2016 07:35 AM, Anthony PERARD wrote:
> >>>> On Wed, Jul 27, 2016 at 12:08:04PM +0100, Anthony PERARD wrote:
> >>>>> I can try to describe how OVMF is setting up the memory.
> >>>> From the start of the day:
> >>>> setup gdt
> >>>> cr0 = 0x40000023
> >>> I think this is slightly odd, with bit 30 (cache disable) set. I'd
> >>> suspect that this would affect both Intel and AMD though.
> >>>
> >>> Can you try clearing this bit?
> >> That works...
> >>
> >> I wonder why it does not appear to affect Intel or KVM.
> > Are those bits hard-coded, or are they set based on the hardware
> > that's available?
> >
> > Is it possible that the particular combination of CPUID bits presented
> > by Xen on AMD are causing a different value to be written?
> >
> > Or is it possible that the cache disable bit is being ignored (by Xen)
> > on Intel and KVM?
> 
> If a guest has no hardware, then it has no reason to actually disable
> caches.  We should have logic to catch this an avoid actually disabling
> caches when the guest asks for it.

For KVM/QEMU, the OVMF binary is loaded in a 'CFI parallel flash', which
is map via mmio, so it is executed from a device memory. The memory
region is marked as rom_device.

-- 
Anthony PERARD

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel