[Xen-devel] [PATCH] x86/xen: fix xsave capability setting

[Juergen Gross <jgross@xxxxxxxx>]

Commit 690b7f10b4f9f ("x86/xen: use capabilities instead of fake cpuid
values for xsave") introduced a regression as it tried to make use of
the fixup feature before it being available.

Fall back to the old variant testing via cpuid().

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/xen/enlighten_pv.c | 30 ++++++++----------------------
 1 file changed, 8 insertions(+), 22 deletions(-)

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index a732bc2b9dfc..a1af4f68278f 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -277,26 +277,15 @@ static bool __init xen_check_mwait(void)
 
 static bool __init xen_check_xsave(void)
 {
-       unsigned int err, eax, edx;
+       unsigned int cx, xsave_mask;
 
-       /*
-        * Xen 4.0 and older accidentally leaked the host XSAVE flag into guest
-        * view, despite not being able to support guests using the
-        * functionality. Probe for the actual availability of XSAVE by seeing
-        * whether xgetbv executes successfully or raises #UD.
-        */
-       asm volatile("1: .byte 0x0f,0x01,0xd0\n\t" /* xgetbv */
-                    "xor %[err], %[err]\n"
-                    "2:\n\t"
-                    ".pushsection .fixup,\"ax\"\n\t"
-                    "3: movl $1,%[err]\n\t"
-                    "jmp 2b\n\t"
-                    ".popsection\n\t"
-                    _ASM_EXTABLE(1b, 3b)
-                    : [err] "=r" (err), "=a" (eax), "=d" (edx)
-                    : "c" (0));
+       cx = cpuid_ecx(1);
 
-       return err == 0;
+       xsave_mask = (1 << (X86_FEATURE_XSAVE % 32)) |
+                    (1 << (X86_FEATURE_OSXSAVE % 32));
+
+       /* Xen will set CR4.OSXSAVE if supported and not disabled by force */
+       return (cx & xsave_mask) == xsave_mask;
 }
 
 static void __init xen_init_capabilities(void)
@@ -317,10 +306,7 @@ static void __init xen_init_capabilities(void)
        else
                setup_clear_cpu_cap(X86_FEATURE_MWAIT);
 
-       if (xen_check_xsave()) {
-               setup_force_cpu_cap(X86_FEATURE_XSAVE);
-               setup_force_cpu_cap(X86_FEATURE_OSXSAVE);
-       } else {
+       if (!xen_check_xsave()) {
                setup_clear_cpu_cap(X86_FEATURE_XSAVE);
                setup_clear_cpu_cap(X86_FEATURE_OSXSAVE);
        }
-- 
2.12.0


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