linux-kernel - Re: [PATCH v2 3/3] KVM: VMX: make MSR bitmaps per-VCPU

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Date:   Wed, 31 Jan 2018 12:37:09 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        David Woodhouse <dwmw@...zon.co.uk>,
        KarimAllah Ahmed <karahmed@...zon.de>
Subject: Re: [PATCH v2 3/3] KVM: VMX: make MSR bitmaps per-VCPU

On 30/01/2018 11:23, Radim Krčmář wrote:
> 2018-01-27 09:50+0100, Paolo Bonzini:
>> Place the MSR bitmap in struct loaded_vmcs, and update it in place
>> every time the x2apic or APICv state can change.  This is rare and
>> the loop can handle 64 MSRs per iteration, in a similar fashion as
>> nested_vmx_prepare_msr_bitmap.
>>
>> This prepares for choosing, on a per-VM basis, whether to intercept
>> the SPEC_CTRL and PRED_CMD MSRs.
>>
>> Suggested-by: Jim Mattson <jmattson@...gle.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> @@ -10022,7 +10043,7 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>>  	int msr;
>>  	struct page *page;
>>  	unsigned long *msr_bitmap_l1;
>> -	unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.msr_bitmap;
>> +	unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.vmcs02.msr_bitmap;
> 
> The physical address of the nested msr_bitmap is never loaded into vmcs.
> 
> The resolution you provided had extra hunk in prepare_vmcs02_full():
> 
> +	vmcs_write64(MSR_BITMAP, __pa(vmx->nested.vmcs02.msr_bitmap));
> 
> I have queued that as:
> 
> +	if (cpu_has_vmx_msr_bitmap())
> +		vmcs_write64(MSR_BITMAP, __pa(vmx->nested.vmcs02.msr_bitmap));

Hmm you're right, it should be in prepare_vmcs02() here (4.15-based),
and then moved to prepare_vmcs02_full() as part of the conflict resolution.

I'll send a v3.

Paolo