[xen master] x86/nSVM: Expose the FlushByASID CPU feature support to L1 guests

[patchbot@xxxxxxx]

commit cd36ac1c7716e6d63fe020cc357691cbb0f2e1af
Author:     Abdelkareem Abdelsaamad <abdelkareem.abdelsaamad@xxxxxxxxxx>
AuthorDate: Thu Aug 27 08:07:01 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Aug 27 08:07:01 2026 +0200

    x86/nSVM: Expose the FlushByASID CPU feature support to L1 guests
    
    Some hypervisors explicitly require FlushByASID capability to function under
    nested virtualization; otherwise, they fail to boot or run nested guests, 
see
    see Xen's start_nested_svm and (1) and (2). To ensure functional nested
    virtualization support and allow L1 hypervisors to correctly detect this
    hardware capability, extend the exposed HVM CPU policy to surface the
    FlushByASID CPU feature to the L1 guests when nestedhvm is enabled.
    
    (1) VMWare (Launchpad Bug #2008583) 
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2008583
    (2) 
https://lore.kernel.org/all/b9915c9c-4cf6-051a-2d91-44cc6380f455@xxxxxxxxxxx/T/#u
    
    Signed-off-by: Abdelkareem Abdelsaamad <abdelkareem.abdelsaamad@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/cpu-policy.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index eddcd9778f..48a3185eed 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -843,6 +843,7 @@ static void __init calculate_hvm_max_policy(void)
         p->extd.raw[0xa].d &= ((1u << SVM_FEATURE_NPT) |
                                (1u << SVM_FEATURE_LBRV) |
                                (1u << SVM_FEATURE_NRIPS) |
+                               (1u << SVM_FEATURE_FLUSHBYASID) |
                                (1u << SVM_FEATURE_PAUSEFILTER) |
                                (1u << SVM_FEATURE_DECODEASSISTS));
         /* Enable features which are always emulated. */
--
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