[xen master] x86/xstate: Fix initialisation of XSS cache

[patchbot@xxxxxxx]

commit 9e6dbbe8bf400aacb99009ddffa91d2a0c312b39
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed May 22 17:23:54 2024 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Jun 19 13:00:06 2024 +0100

    x86/xstate: Fix initialisation of XSS cache
    
    The clobbering of this_cpu(xcr0) and this_cpu(xss) to architecturally 
invalid
    values is to force the subsequent set_xcr0() and set_msr_xss() to reload the
    hardware register.
    
    While XCR0 is reloaded in xstate_init(), MSR_XSS isn't.  This causes
    get_msr_xss() to return the invalid value, and logic of the form:
    
        old = get_msr_xss();
        set_msr_xss(new);
        ...
        set_msr_xss(old);
    
    to try and restore said invalid value.
    
    The architecturally invalid value must be purged from the cache, meaning the
    hardware register must be written at least once.  This in turn highlights 
that
    the invalid value must only be used in the case that the hardware register 
is
    available.
    
    Fixes: f7f4a523927f ("x86/xstate: reset cached register values on resume")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/arch/x86/xstate.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 99cedb4f5e..7578814796 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -641,13 +641,6 @@ void xstate_init(struct cpuinfo_x86 *c)
         return;
     }
 
-    /*
-     * Zap the cached values to make set_xcr0() and set_msr_xss() really
-     * write it.
-     */
-    this_cpu(xcr0) = 0;
-    this_cpu(xss) = ~0;
-
     cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
     feature_mask = (((u64)edx << 32) | eax) & XCNTXT_MASK;
     BUG_ON(!valid_xcr0(feature_mask));
@@ -657,8 +650,19 @@ void xstate_init(struct cpuinfo_x86 *c)
      * Set CR4_OSXSAVE and run "cpuid" to get xsave_cntxt_size.
      */
     set_in_cr4(X86_CR4_OSXSAVE);
+
+    /*
+     * Zap the cached values to make set_xcr0() and set_msr_xss() really write
+     * the hardware register.
+     */
+    this_cpu(xcr0) = 0;
     if ( !set_xcr0(feature_mask) )
         BUG();
+    if ( cpu_has_xsaves )
+    {
+        this_cpu(xss) = ~0;
+        set_msr_xss(0);
+    }
 
     if ( bsp )
     {
--
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