linux-kernel - Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc

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Message-ID: <0220f903-2915-f072-b1da-0b58fc07f416@redhat.com>
Date:   Tue, 25 May 2021 18:15:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Stamatis, Ilias" <ilstam@...zon.com>
Cc:     "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "zamsden@...il.com" <zamsden@...il.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and
 decache_tsc_multiplier()

On 25/05/21 17:58, Sean Christopherson wrote:
>> The right place for the hw multiplier
>> field to be updated is inside set_tsc_khz() in common code when the ratio
>> changes.

Sort of, the problem is that you have two VMCS's to update.  If properly 
fixed, the cache is useful to fix the issue with KVM_SET_TSC_KHZ needing 
to update both of them.  For that to work, you'd have to move the cache 
to struct loaded_vmcs.

So you can:

1) move the cached tsc_ratio to struct loaded_vmcs

2) add a function in common code (update_tsc_parameters or something 
like that) to update both the offset and the ratio depending on 
is_guest_mode()

3) call that function from nested vmentry/vmexit

And at that point the cache will do its job and figure out whether a 
vmwrite is needed, on both vmentry and vmexit.

I actually like the idea of storing the expected value in kvm_vcpu and 
the current value in loaded_vmcs.  We might use it for other things such 
as reload_vmcs01_apic_access_page perhaps.

Paolo

>> However, this requires adding another vendor callback etc. As all
>> this is further refactoring I believe it's better to leave this series as is -
>> ie only touching code that is directly related to nested TSC scaling and not
>> try to do everything as part of the same series.
> But it directly impacts your code, e.g. the nested enter/exit flows would need
> to dance around the decache silliness.  And I believe it even more directly
> impacts this series: kvm_set_tsc_khz() fails to handle the case where userspace
> invokes KVM_SET_TSC_KHZ while L2 is active.
>