Re: [SeaBIOS] Re: [PATCH 4/4] be less conservative with the 64bit pci io window

[Igor Mammedov]

On Wed, 23 Nov 2022 11:25:08 +0100
Gerd Hoffmann <kraxel@redhat.com> wrote:

> On Tue, Nov 22, 2022 at 01:43:16PM -0500, Kevin O'Connor wrote:
> > On Mon, Nov 21, 2022 at 11:32:13AM +0100, Gerd Hoffmann wrote:  
> > > Current seabios code will only enable and use the 64bit pci io window in
> > > case it runs out of space in the 32bit pci mmio window below 4G.
> > > 
> > > This patch will also enable the 64bit pci io window when
> > >   (a) RAM above 4G is present, and
> > >   (b) the physical address space size is known, and
> > >   (c) seabios is running on a 64bit capable processor.
> > > 
> > > This operates with the assumption that guests which are ok with memory
> > > above 4G most likely can handle mmio above 4G too.  
> > 
> > Thanks.  In general, the series looks good to me.  Can you elaborate
> > on the background to this change though?  It sounds like there is a
> > (small) risk of a regression, so I think it would be good to have a
> > high level understanding of what is driving this memory reorg.  
> 
> Well, the idea is to adapt to the world moving forward.  Running a
> 64-bit capable OS is standard these days, and the resources needed
> by devices (especially GPUs) are becoming larger and larger.
> 
> Yes, there is the risk that (old) guests are unhappy with their
> PCI bars suddenly being mapped above 4G.  Can happen only in case
> seabios handles pci initialization (i.e. when running on qemu,
> otherwise coreboot initializes the pci bars).  I hope the memory
> check handles the 'old guest' case: when the guest can't handle
> addresses above 4G it is unlikely that qemu is configured to have
> memory mapped above 4G ...

does it break 32-bit PAE enabled guests
(which can have more then 4Gb RAM configured)?

> 
> take care,
>   Gerd
> 
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