[PATCH v2] x86/vmx: Fixup removals of MSR load/save list entries

From: Igor Druzhinin <igor.druzhinin@citrix.com>
To: xen-devel@lists.xenproject.org
Cc: Igor Druzhinin <igor.druzhinin@citrix.com>,
	kevin.tian@intel.com, wei.liu2@citrix.com, jbeulich@suse.com,
	andrew.cooper3@citrix.com, jun.nakajima@intel.com,
	roger.pau@citrix.com
Subject: [PATCH v2] x86/vmx: Fixup removals of MSR load/save list entries
Date: Thu, 4 Apr 2019 17:25:10 +0100	[thread overview]
Message-ID: <1554395110-22095-1-git-send-email-igor.druzhinin@citrix.com> (raw)

Commit 540d5422 ("x86/vmx: Support removing MSRs from the host/guest
load/save lists") introduced infrastructure finally exposed by
commit fd32dcfe ("x86/vmx: Don't leak EFER.NXE into guest context")
that led to a functional regression on Harpertown and earlier cores
(Gen 1 VT-x) due to MSR count being incorrectly set in VMCS.
As the result, as soon as guest EFER becomes equal to Xen EFER
(which eventually happens in almost every 64-bit VM) and its MSR
entry is supposed to be removed, a stale version of EFER is loaded
into a guest instead causing almost immediate guest failure.

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
---
Changes in v2:
* better commit description as suggested
---
 xen/arch/x86/hvm/vmx/vmcs.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
index 74f2a08..45d1849 100644
--- a/xen/arch/x86/hvm/vmx/vmcs.c
+++ b/xen/arch/x86/hvm/vmx/vmcs.c
@@ -1490,15 +1490,15 @@ int vmx_del_msr(struct vcpu *v, uint32_t msr, enum vmx_msr_list_type type)
     switch ( type )
     {
     case VMX_MSR_HOST:
-        __vmwrite(VM_EXIT_MSR_LOAD_COUNT, vmx->host_msr_count--);
+        __vmwrite(VM_EXIT_MSR_LOAD_COUNT, --vmx->host_msr_count);
         break;
 
     case VMX_MSR_GUEST:
-        __vmwrite(VM_EXIT_MSR_STORE_COUNT, vmx->msr_save_count--);
+        __vmwrite(VM_EXIT_MSR_STORE_COUNT, --vmx->msr_save_count);
 
         /* Fallthrough */
     case VMX_MSR_GUEST_LOADONLY:
-        __vmwrite(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_load_count--);
+        __vmwrite(VM_ENTRY_MSR_LOAD_COUNT, --vmx->msr_load_count);
         break;
     }
 
-- 
2.7.4


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