Re: [PATCH] hw/i386/acpi: Set PCAT_COMPAT bit only when pic is not disabled

[Kirill A. Shutemov]

On Wed, Apr 03, 2024 at 10:03:15AM +0800, Xiaoyao Li wrote:
> On 4/2/2024 10:31 PM, Michael S. Tsirkin wrote:
> > On Tue, Apr 02, 2024 at 09:18:44PM +0800, Xiaoyao Li wrote:
> > > On 4/2/2024 6:02 PM, Michael S. Tsirkin wrote:
> > > > On Tue, Apr 02, 2024 at 04:25:16AM -0400, Xiaoyao Li wrote:
> > > > > Set MADT.FLAGS[bit 0].PCAT_COMPAT based on x86ms->pic.
> > > > > 
> > > > > Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> > > > 
> > > > Please include more info in the commit log:
> > > > what is the behaviour you observe, why it is wrong,
> > > > how does the patch fix it, what is guest behaviour
> > > > before and after.
> > > 
> > > Sorry, I thought it was straightforward.
> > > 
> > > A value 1 of PCAT_COMPAT (bit 0) of MADT.Flags indicates that the system
> > > also has a PC-AT-compatible dual-8259 setup, i.e., the PIC.
> > > 
> > > When PIC is not enabled for x86 machine, the PCAT_COMPAT bit needs to be
> > > cleared. Otherwise, the guest thinks there is a present PIC even it is
> > > booted with pic=off on QEMU.
> > > 
> > > (I haven't seen real issue from Linux guest. The user of PIC inside guest
> > > seems only the pit calibration. Whether pit calibration is triggered 
> > > depends
> > > on other things. But logically, current code is wrong, we need to fix it
> > > anyway.
> > > 
> > > @Isaku, please share more info if you have)
> > > 
> 
> + Kirill,
> 
> It seems to have issue with legacy irqs with PCAT_COMPAT set 1 while no PIC
> on QEMU side. Kirill, could you elaborate it?

TDX guest cannot support PIC because the platform doesn't allow direct
interrupt injection, only posted interrupts.

For TDX guest kernel we had a patch[1] that forces no-PIC, but it is not
upstreamable as it is a hack.

I looked around to find The Right Way™ to archive the same effect and
discovered that we only have PIC ops hooked up because kernel bypasses[2]
PIC enumeration because PCAT_COMPAT is set. Which is wrong for TDX guest
or other platforms without PIC.

I am not aware about any user-visible issues due to it, but maybe they are
just not discovered yet.

[1] 
https://lore.kernel.org/linux-kernel/b29f00c1eb5cff585ec2b999b69923c13418ecc4.1619458733.git.sathyanarayanan.kuppuswamy@linux.intel.com/
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/i8259.c#n322

-- 
  Kiryl Shutsemau / Kirill A. Shutemov