linux-kernel - Re: [PATCH v3] KVM: VMX: do not disable interception for MSR_IA32_SPEC

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Message-ID: <3C8F5313-2830-46E3-A512-CFA4A24C24D7@nutanix.com>
Date:   Wed, 25 May 2022 17:14:38 +0000
From:   Jon Kohler <jon@...anix.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Jon Kohler <jon@...anix.com>, Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] KVM: VMX: do not disable interception for
 MSR_IA32_SPEC_CTRL on eIBRS



> On May 25, 2022, at 1:04 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> 
> On Fri, May 20, 2022, Jon Kohler wrote:
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 610355b9ccce..1c725d17d984 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -2057,20 +2057,32 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> 			return 1;
>> 
>> 		vmx->spec_ctrl = data;
>> -		if (!data)
>> +
>> +		/*
>> +		 * Disable interception on the first non-zero write, unless the
>> +		 * guest is hosted on an eIBRS system and setting only
> 
> The "unless guest is hosted on an eIBRS system" blurb is wrong and doesn't match

Ah right, thanks for catching that

> the code.  Again, it's all about whether eIBRS is advertised to the guest.  With
> some other minor tweaking to wrangle the comment to 80 chars...

RE 80 chars - quick question (and forgive the silly question here), but how are you
counting that? I’ve got my editor cutting at 79 cols, where tab size is accounted
for as 4 cols, so the longest line on my side for this patch is 72-73 or so.

These also pass the checkpatch.pl script as well, so I just want to make sure
going forward I’m formatting them appropriately.

Let me know and I’ll incorporate all of this into a v4 after I hear back :)

> 
> 		/*
>                 * Disable interception on the first non-zero write, except if
> 		 * eIBRS is advertised to the guest and the guest is enabling
> 		 * _only_ IBRS.  On eIBRS systems, kernels typically set IBRS
> 		 * once at boot and never touch it post-boot.  All other bits,
> 		 * and IBRS on non-eIBRS systems, are often set on a per-task
> 		 * basis, i.e. change frequently, so the benefit of avoiding
> 		 * VM-exits during guest context switches outweighs the cost of
> 		 * RDMSR on every VM-Exit to save the guest's value.
> 		 */

Thanks for the suggestion, this text works for me

> 
>> +		 * SPEC_CTRL_IBRS, which is typically set once at boot and never
> 
> Uber nit, when it makes sense, avoid regurgitating the code verbatim, e.g. refer
> to "setting SPEC_CTRL_IBRS" as "enabling IBRS".  That little bit of abstraction
> can sometimes help unfamiliar readers understand the effect of the code, whereas
> copy+pasting bits of the code doesn't provide any additional context.

Ok thanks for this as well, I appreciate the feedback!

> 
>> +		 * touched again.  All other bits are often set on a per-task
>> +		 * basis, i.e. may change frequently, so the benefit of avoiding
>> +		 * VM-exits during guest context switches outweighs the cost of
>> +		 * RDMSR on every VM-Exit to save the guest's value.
>> +		 */
>> +		if (!data ||
>> +			(data == SPEC_CTRL_IBRS &&
>> +			 (vcpu->arch.arch_capabilities & ARCH_CAP_IBRS_ALL)))
> 
> Align the two halves of the logical-OR, i.e.
> 
> 		if (!data ||
> 		    (data == SPEC_CTRL_IBRS &&
> 		     (vcpu->arch.arch_capabilities & ARCH_CAP_IBRS_ALL)))
> 			break;

Ack on this