[Xen-devel] [PATCH v2] VMX: Eliminate cr3 save/loading exiting when UG enabled

[Yang Zhang <yang.z.zhang@xxxxxxxxx>]

From: Yang Zhang <yang.z.zhang@xxxxxxxxx>

With the feature of unrestricted guest, there should no vmexit
be triggered when guest accesses the cr3 in non-paging mode.

Signed-off-by: Yang Zhang <yang.z.zhang@xxxxxxxxx>
---
changes in v2:
Fix the guest boot failure on non-UG platform.

The previous patch doesn't consider the non-UG platform and will cause guest 
boot
failure on non-ug platform. During developing this patch, i was thinking whether
the reading of GUEST_CR3 is necessary on each vmexit. I thought the answer 
should
be no. But current implemention relies the {hw,guest}_cr3 in many places and it
hard to do a cleanup.(If anyone interesting, welcome :))

BTW: this patch will improve performance, especially for context switching 
intensive
workload, we saw about 1% improvement. So please backport it to 4.3.
---
 xen/arch/x86/hvm/vmx/vmx.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index dfff628..f6409d6 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1157,7 +1157,7 @@ static void vmx_update_guest_cr(struct vcpu *v, unsigned 
int cr)
             uint32_t cr3_ctls = (CPU_BASED_CR3_LOAD_EXITING |
                                  CPU_BASED_CR3_STORE_EXITING);
             v->arch.hvm_vmx.exec_control &= ~cr3_ctls;
-            if ( !hvm_paging_enabled(v) )
+            if ( !hvm_paging_enabled(v) && !vmx_unrestricted_guest(v) )
                 v->arch.hvm_vmx.exec_control |= cr3_ctls;
 
             /* Trap CR3 updates if CR3 memory events are enabled. */
@@ -1231,7 +1231,7 @@ static void vmx_update_guest_cr(struct vcpu *v, unsigned 
int cr)
     case 3:
         if ( paging_mode_hap(v->domain) )
         {
-            if ( !hvm_paging_enabled(v) )
+            if ( !hvm_paging_enabled(v) && !vmx_unrestricted_guest(v) )
                 v->arch.hvm_vcpu.hw_cr[3] =
                     v->domain->arch.hvm_domain.params[HVM_PARAM_IDENT_PT];
             vmx_load_pdptrs(v);
@@ -2487,10 +2487,11 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
 
     hvm_invalidate_regs_fields(regs);
 
-    if ( paging_mode_hap(v->domain) && hvm_paging_enabled(v) )
+    if ( paging_mode_hap(v->domain) )
     {
         __vmread(GUEST_CR3, &v->arch.hvm_vcpu.hw_cr[3]);
-        v->arch.hvm_vcpu.guest_cr[3] = v->arch.hvm_vcpu.hw_cr[3];
+        if ( vmx_unrestricted_guest(v) || hvm_paging_enabled(v) )
+            v->arch.hvm_vcpu.guest_cr[3] = v->arch.hvm_vcpu.hw_cr[3];
     }
 
     __vmread(VM_EXIT_REASON, &exit_reason);
-- 
1.7.1


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