[xen staging] SVM: avoid VMSAVE in ctxt-switch-to

[patchbot@xxxxxxx]

commit a7f0831e58bf4681d710e9a029644b6fa07b7cb0
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Oct 20 08:54:59 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Oct 20 08:54:59 2020 +0200

    SVM: avoid VMSAVE in ctxt-switch-to
    
    Of the state saved by the insn and reloaded by the corresponding VMLOAD
    - TR and syscall state are invariant while having Xen's state loaded,
    - sysenter is unused altogether by Xen,
    - FS, GS, and LDTR are not used by Xen and get suitably set in PV
      context switch code.
    Note that state is suitably populated in _svm_cpu_up(); a minimal
    respective assertion gets added.
    
    Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/hvm/svm/svm.c | 3 ++-
 xen/include/asm-x86/desc.h | 9 +++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 9b65445577..cfea5b5523 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -984,7 +984,6 @@ static void svm_ctxt_switch_to(struct vcpu *v)
 
     svm_restore_dr(v);
 
-    svm_vmsave_pa(per_cpu(host_vmcb, cpu));
     vmcb->cleanbits.raw = 0;
     svm_tsc_ratio_load(v);
 
@@ -1609,6 +1608,8 @@ static int _svm_cpu_up(bool bsp)
     /* Initialize OSVW bits to be used by guests */
     svm_host_osvw_init();
 
+    /* Minimal checking that enough CPU setup was done by now. */
+    ASSERT(str() == TSS_SELECTOR);
     svm_vmsave_pa(per_cpu(host_vmcb, cpu));
 
     return 0;
diff --git a/xen/include/asm-x86/desc.h b/xen/include/asm-x86/desc.h
index 24db3e9510..dfa643925f 100644
--- a/xen/include/asm-x86/desc.h
+++ b/xen/include/asm-x86/desc.h
@@ -236,6 +236,15 @@ static inline void ltr(unsigned int sel)
     __asm__ __volatile__ ( "ltr %w0" :: "rm" (sel) : "memory" );
 }
 
+static inline unsigned int str(void)
+{
+    unsigned int sel;
+
+    __asm__ ( "str %0" : "=r" (sel) );
+
+    return sel;
+}
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* __ARCH_DESC_H */
--
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