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: 64b1da5a2f ("x86/svm: Use flush-by-asid when available")
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

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;
 }
 
 /*