[xen master] x86/xstate: reset cached register values on resume

[patchbot@xxxxxxx]

commit f7f4a523927fa4c7598e4647a16bc3e3cf8009d0
Author:     Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
AuthorDate: Thu Nov 4 14:42:37 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Nov 4 14:42:37 2021 +0100

    x86/xstate: reset cached register values on resume
    
    set_xcr0() and set_msr_xss() use cached value to avoid setting the
    register to the same value over and over. But suspend/resume implicitly
    reset the registers and since percpu areas are not deallocated on
    suspend anymore, the cache gets stale.
    Reset the cache on resume, to ensure the next write will really hit the
    hardware. Choose value 0, as it will never be a legitimate write to
    those registers - and so, will force write (and cache update).
    
    Note the cache is used io get_xcr0() and get_msr_xss() too, but:
    - set_xcr0() is called few lines below in xstate_init(), so it will
      update the cache with appropriate value
    - get_msr_xss() is not used anywhere - and thus not before any
      set_msr_xss() that will fill the cache
    
    Fixes: aca2a985a55a "xen: don't free percpu areas during suspend"
    Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>
---
 xen/arch/x86/xstate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
index 6aaf9a2f15..3b32bdc51c 100644
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -642,6 +642,13 @@ 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));
--
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