linux-kernel - Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM

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Date:   Tue, 18 Jan 2022 18:12:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after
 KVM_RUN for CPU hotplug

On 1/18/22 17:53, Sean Christopherson wrote:
>>
>> and I think we should redo all or most of kvm_update_cpuid_runtime
>> the same way.
> Please no.  xstate_required_size() requires multiple host CPUID calls, and glibc
> does CPUID.0xD.0x0 and CPUID.0xD.0x1 as part of its initialization, i.e. launching
> a new userspace process in the guest will see additional performance overhread due
> to KVM dynamically computing the XSAVE size instead of caching it based on vCPU
> state.  Nested virtualization would be especially painful as every one of those
> "host" CPUID invocations will trigger and exit from L1=>L0.
> 

Agreed, but all of the required data is by Linux cached in 
xstate_offsets, xstate_sizes and xstate_comp_offsets; moving 
xstate_required_size to xstate.c and skipping the host CPUID would 
probably be a good idea nevertheless.

Paolo