[xen stable-4.16] x86/CPUID: surface suitable value in EBX of XSTATE subleaf 1

[patchbot@xxxxxxx]

commit e8882bcfe35520e950ba60acd6e67e65f1ce90a8
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Oct 11 14:59:26 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 11 14:59:26 2022 +0200

    x86/CPUID: surface suitable value in EBX of XSTATE subleaf 1
    
    While the SDM isn't very clear about this, our present behavior make
    Linux 5.19 unhappy. As of commit 8ad7e8f69695 ("x86/fpu/xsave: Support
    XSAVEC in the kernel") they're using this CPUID output also to size
    the compacted area used by XSAVEC. Getting back zero there isn't really
    liked, yet for PV that's the default on capable hardware: XSAVES isn't
    exposed to PV domains.
    
    Considering that the size reported is that of the compacted save area,
    I view Linux'es assumption as appropriate (short of the SDM properly
    considering the case). Therefore we need to populate the field also when
    only XSAVEC is supported for a guest.
    
    Fixes: 460b9a4b3630 ("x86/xsaves: enable xsaves/xrstors for hvm guest")
    Fixes: 8d050ed1097c ("x86: don't expose XSAVES capability to PV guests")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: c3bd0b83ea5b7c0da6542687436042eeea1e7909
    master date: 2022-08-24 14:23:59 +0200
---
 xen/arch/x86/cpuid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index ff335f1639..a647331f47 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -1060,7 +1060,7 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
         switch ( subleaf )
         {
         case 1:
-            if ( p->xstate.xsaves )
+            if ( p->xstate.xsavec || p->xstate.xsaves )
             {
                 /*
                  * TODO: Figure out what to do for XSS state.  VT-x manages
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.16