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...@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.kuppusw...@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
