linux-kernel - Re: [PATCH v2 1/3] KVM: x86: dump_vmcs should not assume GUEST_IA32

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Date:   Tue, 23 Feb 2021 14:51:19 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Edmondson <david.edmondson@...cle.com>
Cc:     linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: dump_vmcs should not assume
 GUEST_IA32_EFER is valid

On Fri, Feb 19, 2021, David Edmondson wrote:
> If the VM entry/exit controls for loading/saving MSR_EFER are either
> not available (an older processor or explicitly disabled) or not
> used (host and guest values are the same), reading GUEST_IA32_EFER
> from the VMCS returns an inaccurate value.
> 
> Because of this, in dump_vmcs() don't use GUEST_IA32_EFER to decide
> whether to print the PDPTRs - do so if the EPT is in use and CR4.PAE
> is set.

This isn't necessarily correct either.  In a way, it's less correct as PDPTRs
are more likely to be printed when they shouldn't, assuming most guests are
64-bit guests.  It's annoying to calculate the effective guest EFER, but so
awful that it's worth risking confusion over PDTPRs.

> Fixes: 4eb64dce8d0a ("KVM: x86: dump VMCS on invalid entry")
> Signed-off-by: David Edmondson <david.edmondson@...cle.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index eb69fef57485..818051c9fa10 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5759,7 +5759,6 @@ void dump_vmcs(void)
>  	u32 vmentry_ctl, vmexit_ctl;
>  	u32 cpu_based_exec_ctrl, pin_based_exec_ctrl, secondary_exec_control;
>  	unsigned long cr4;
> -	u64 efer;
>  
>  	if (!dump_invalid_vmcs) {
>  		pr_warn_ratelimited("set kvm_intel.dump_invalid_vmcs=1 to dump internal KVM state.\n");
> @@ -5771,7 +5770,6 @@ void dump_vmcs(void)
>  	cpu_based_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
>  	pin_based_exec_ctrl = vmcs_read32(PIN_BASED_VM_EXEC_CONTROL);
>  	cr4 = vmcs_readl(GUEST_CR4);
> -	efer = vmcs_read64(GUEST_IA32_EFER);
>  	secondary_exec_control = 0;
>  	if (cpu_has_secondary_exec_ctrls())
>  		secondary_exec_control = vmcs_read32(SECONDARY_VM_EXEC_CONTROL);
> @@ -5784,8 +5782,7 @@ void dump_vmcs(void)
>  	       cr4, vmcs_readl(CR4_READ_SHADOW), vmcs_readl(CR4_GUEST_HOST_MASK));
>  	pr_err("CR3 = 0x%016lx\n", vmcs_readl(GUEST_CR3));
>  	if ((secondary_exec_control & SECONDARY_EXEC_ENABLE_EPT) &&
> -	    (cr4 & X86_CR4_PAE) && !(efer & EFER_LMA))
> -	{
> +	    (cr4 & X86_CR4_PAE)) {
>  		pr_err("PDPTR0 = 0x%016llx  PDPTR1 = 0x%016llx\n",
>  		       vmcs_read64(GUEST_PDPTR0), vmcs_read64(GUEST_PDPTR1));
>  		pr_err("PDPTR2 = 0x%016llx  PDPTR3 = 0x%016llx\n",
> @@ -5811,7 +5808,8 @@ void dump_vmcs(void)
>  	if ((vmexit_ctl & (VM_EXIT_SAVE_IA32_PAT | VM_EXIT_SAVE_IA32_EFER)) ||
>  	    (vmentry_ctl & (VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_IA32_EFER)))
>  		pr_err("EFER =     0x%016llx  PAT = 0x%016llx\n",
> -		       efer, vmcs_read64(GUEST_IA32_PAT));
> +		       vmcs_read64(GUEST_IA32_EFER),
> +		       vmcs_read64(GUEST_IA32_PAT));
>  	pr_err("DebugCtl = 0x%016llx  DebugExceptions = 0x%016lx\n",
>  	       vmcs_read64(GUEST_IA32_DEBUGCTL),
>  	       vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS));
> -- 
> 2.30.0
>