Re: [PATCH v4 8/9] target/i386/mshv: filter out CET bits in cpuid

[Magnus Kulke <magnuskulke@linux.microsoft.com>]

On Tue, Apr 14, 2026 at 06:10:19PM +0200, Paolo Bonzini wrote:
> I don't understand this, but it seems to be a Linux bug.  Don't *all* xsave
> features cause changes to CPUID[0xD,1].EBX?

oh, you're right, thx. the CPUID[0xD,1].EBX responses (also CPUID[0xD,0])
returned wrong responses in all cases the introduction of CET in QEMU
probably just triggered some overflow, but it was broken before.

We cannot statically register size responses for xsave features that a
guest can dynamically enable/disable.

I was planning to implement a dynamic handler for the xsave size
responses anyway, but maybe there's an easier way: We can provide an ebx
mask = 0 in the payload when we register the cpuid responses at the
hypervisor. If we do that for CPUID[0xD,{0,1}], the hypervisor should
return the answer based on the guest state (XCR0,XSS).

I'll test this approach and probably post a new revision with one of
those options.