linux-kernel - Re: [PATCH 1/3] kvm: vmx: Add IA32_FLUSH

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Message-ID: <301c7527-6319-b993-f43f-dc61b9af4b34@redhat.com>
Date:   Mon, 20 Mar 2023 16:40:13 +0100
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Nathan Chancellor <nathan@...nel.org>, kvm@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>,
        Ben Serebrin <serebrin@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] kvm: vmx: Add IA32_FLUSH_CMD guest support



Am 20/03/2023 um 15:53 schrieb Sean Christopherson:
> On Fri, Mar 17, 2023, Pawan Gupta wrote:
>> On Fri, Mar 17, 2023 at 04:14:01PM -0700, Nathan Chancellor wrote:
>>> On Fri, Mar 17, 2023 at 03:53:45PM -0700, Pawan Gupta wrote:
>>>> On Fri, Mar 17, 2023 at 12:04:32PM -0700, Nathan Chancellor wrote:
>>>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>>>>> index c788aa382611..9a78ea96a6d7 100644
>>>>>> --- a/arch/x86/kvm/vmx/vmx.c
>>>>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>>>>> @@ -2133,6 +2133,39 @@ static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated
>>>>>>  	return debugctl;
>>>>>>  }
>>>>>>  
>>>>>> +static int vmx_set_msr_ia32_cmd(struct kvm_vcpu *vcpu,
>>>>>> +				struct msr_data *msr_info,
>>>>>> +				bool guest_has_feat, u64 cmd,
>>>>>> +				int x86_feature_bit)
>>>>>> +{
>>>>>> +	if (!msr_info->host_initiated && !guest_has_feat)
>>>>>> +		return 1;
>>>>>> +
>>>>>> +	if (!(msr_info->data & ~cmd))
>>>>
>>>> Looks like this is doing a reverse check. Shouldn't this be as below:
>>>
>>> That diff on top of next-20230317 appears to resolve the issue for me
>>> and my L1 guest can spawn an L2 guest without any issues (which is the
>>> extent of my KVM testing).
>>
>> Great!
>>
>>> Is this a problem for the SVM version? It has the same check it seems,
>>> although I did not have any issues on my AMD test platform (but I guess
>>> that means that the system has the support?).
>>
>> IIUC, SVM version also needs to be fixed.
> 
> Yeah, looks that way.  If we do go this route, can you also rename "cmd" to something
> like "allowed_mask"?  It took me far too long to understand what "cmd" represents.
> 
>>> I assume this will just be squashed into the original change but if not:
>>
>> Thats what I think, and if its too late to be squashed I will send a
>> formal patch. Maintainers?
> 
> Honestly, I'd rather revert the whole mess and try again.  

The patches obviously
> weren't tested,
Well... no. They were tested. Call it wrongly tested, badly tested,
whatever you want but don't say "obviously weren't tested". I even asked
you in a private email why the cpu flag was visible in Linux and not in
rhel when using the same machine.

So again, my bad with these patches, I sincerely apologize but I would
prefer that you think I don't know how to test this stuff rather than
say that I carelessly sent something without checking :)

About the rest of the email: whatever you decide is fine for me.

Thank you,
Emanuele

 and the entire approach (that was borrowed from the existing
> MSR_IA32_PRED_CMD code) makes no sense.
> 
> The MSRs are write-only command registers, i.e. don't have a persistent value.
> So unlike MSR_IA32_SPEC_CTRL (which I suspect was the source for the copy pasta),
> where KVM needs to track the guest value, there are no downsides to disabling
> interception of the MSRs.  
> 
> Manually checking the value written by the guest or host userspace is similarly
> ridiculous.  The MSR is being exposed directly to the guest, i.e. after the first
> access, the guest can throw any value at bare metal anyways, so just do wrmsrl_safe()
> and call it good.
> 
> In other words, the common __kvm_set_msr() switch should have something like so,
> 
> 	case MSR_IA32_PRED_CMD:
> 		if (!cpu_feature_enabled(X86_FEATURE_IBPB))
> 			return 1;
> 
> 		if (!msr_info->host_initiated &&
> 		    !guest_cpuid_has(vcpu, guest_has_pred_cmd_msr(vcpu)))
> 			return 1;
> 
> 		ret = !!wrmsrl_safe(MSR_IA32_PRED_CMD, msr_info->data);
> 		break;
> 	case MSR_IA32_FLUSH_CMD:
> 		if (!cpu_feature_enabled(X86_FEATURE_FLUSH_L1D))
> 			return 1;
> 
> 		if (!msr_info->host_initiated &&
> 		    !guest_cpuid_has(vcpu, X86_FEATURE_FLUSH_L1D))
> 			return 1;
> 
> 		ret = !!wrmsrl_safe(MSR_IA32_FLUSH_CMD, msr_info->data);
> 		break;
> 
> with the MSR interception handled in e.g. vmx_vcpu_after_set_cpuid().
> 
> Paolo?
>