[xen master] x86/svm: Always flush TLB using TLB_CTRL_FLUSH_ALL

[patchbot@xxxxxxx]

commit 68871b1f9b8034fef12826610b0f590fd7c212ed
Author:     Teddy Astie <teddy.astie@xxxxxxxxxx>
AuthorDate: Wed Jul 23 15:23:44 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jul 23 15:23:44 2025 +0200

    x86/svm: Always flush TLB using TLB_CTRL_FLUSH_ALL
    
    Xen currently uses an ASID scheme where:
    - ASIDs are cycled where a "TLB flush" is performed
    - When ASIDs wrap around, perform a full TLB flush
    - In exceptional cases, stop using ASIDs
    
    However, the TLB control mode used only flushes the current active ASID of
    the logical processor. Which mean that will supply "stale" ASIDs (not 
flushed),
    because it hasn't been actually flushed (TLB_CTRL_FLUSH_ASID only performs a
    TLB flush of the ASID set in VMCB).
    
    This affects CPUs where flush-by-asid is available (Fam15h (2011) and 
later).
    
    To fix this, always flush all ASIDs even when flush-by-asid is available.
    
    Fixes: 64b1da5a2fcf ("x86/svm: Use flush-by-asid when available")
    Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/hvm/svm/asid.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/asid.c b/xen/arch/x86/hvm/svm/asid.c
index d5f70f8848..0bc1f5aa72 100644
--- a/xen/arch/x86/hvm/svm/asid.c
+++ b/xen/arch/x86/hvm/svm/asid.c
@@ -46,9 +46,8 @@ void svm_asid_handle_vmrun(void)
     if ( vmcb_get_asid(vmcb) != p_asid->asid )
         vmcb_set_asid(vmcb, p_asid->asid);
 
-    vmcb->tlb_control =
-        !need_flush ? TLB_CTRL_NO_FLUSH :
-        cpu_has_svm_flushbyasid ? TLB_CTRL_FLUSH_ASID : TLB_CTRL_FLUSH_ALL;
+    /* We can't rely on TLB_CTRL_FLUSH_ASID as all ASIDs are stale here. */
+    vmcb->tlb_control = need_flush ? TLB_CTRL_FLUSH_ALL : TLB_CTRL_NO_FLUSH;
 }
 
 /*
--
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