linux-kernel - [PATCH] kvm: svm: reset mmu on VCPU reset

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Message-Id: <1442583545-266967-1-git-send-email-imammedo@redhat.com>
Date:	Fri, 18 Sep 2015 15:39:05 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	joro@...tes.org, pbonzini@...hat.com,
	guangrong.xiao@...ux.intel.com, kvm@...r.kernel.org
Subject: [PATCH] kvm: svm: reset mmu on VCPU reset

When INIT/SIPI sequence is sent to VCPU which before that
was in use by OS, VMRUN might fail with:

 KVM: entry failed, hardware error 0xffffffff
 EAX=00000000 EBX=00000000 ECX=00000000 EDX=000006d3
 ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
 EIP=00000000 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
 ES =0000 00000000 0000ffff 00009300
 CS =9a00 0009a000 0000ffff 00009a00
 [...]
 CR0=60000010 CR2=b6f3e000 CR3=01942000 CR4=000007e0
 [...]
 EFER=0000000000000000

with corresponding SVM error:
 KVM: FAILED VMRUN WITH VMCB:
 [...]
 cpl:            0                efer:         0000000000001000
 cr0:            0000000080010010 cr2:          00007fd7fe85bf90
 cr3:            0000000187d0c000 cr4:          0000000000000020
 [...]

What happens is that VCPU state right after offlinig:
CR0: 0x80050033  EFER: 0xd01  CR4: 0x7e0
  -> long mode with CR3 pointing to longmode page tables

and when VCPU gets INIT/SIPI following transition happens
CR0: 0 -> 0x60000010 EFER: 0x0  CR4: 0x7e0
  -> paging disabled with stale CR3

However SVM under the hood puts VCPU in Paged Real Mode*
which effectively translates CR0 0x60000010 -> 80010010 after

   svm_vcpu_reset()
       -> init_vmcb()
           -> kvm_set_cr0()
               -> svm_set_cr0()

but from  kvm_set_cr0() perspective CR0: 0 -> 0x60000010
only caching bits are changed and
commit d81135a57aa6
 ("KVM: x86: do not reset mmu if CR0.CD and CR0.NW are changed")'
regressed svm_vcpu_reset() which relied on MMU being reset.

As result VMRUN after svm_vcpu_reset() tries to run
VCPU in Paged Real Mode with stale MMU context (longmode page tables),
which causes some AMD CPUs** to bail out with VMEXIT_INVALID.

Fix issue by unconditionally resetting MMU context
at init_vmcb() time.

--
* AMD64 Architecture Programmer’s Manual,
    Volume 2: System Programming, rev: 3.25
      15.19 Paged Real Mode
** Opteron 1216

Signed-off-by: Igor Mammedov <imammedo@...hat.com>
---
 arch/x86/kvm/svm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index fdb8cb6..89173af 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1264,6 +1264,7 @@ static void init_vmcb(struct vcpu_svm *svm, bool init_event)
 	 * It also updates the guest-visible cr0 value.
 	 */
 	(void)kvm_set_cr0(&svm->vcpu, X86_CR0_NW | X86_CR0_CD | X86_CR0_ET);
+	kvm_mmu_reset_context(&svm->vcpu);
 
 	save->cr4 = X86_CR4_PAE;
 	/* rdx = ?? */
-- 
1.8.3.1

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