From: Jan Beulich <jbeulich@suse.com>
Date: Tue, 20 Oct 2020 06:54:59 +0000 (+0200)
Subject: SVM: avoid VMSAVE in ctxt-switch-to
X-Git-Tag: 4.15.0-rc1~637
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=a7f0831e58bf4681d710e9a029644b6fa07b7cb0;p=xen.git

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@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
---

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 */