linux-kernel - Re: [PATCH v1 2/4] KVM: x86: Introduce MSR read/write emulation helpers

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Message-ID: <02257a58-393b-4687-803a-e9b3d6bcb786@zytor.com>
Date: Thu, 31 Jul 2025 10:19:48 -0700
From: Xin Li <xin@...or.com>
To: Chao Gao <chao.gao@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, pbonzini@...hat.com,
        seanjc@...gle.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v1 2/4] KVM: x86: Introduce MSR read/write emulation
 helpers

On 7/31/2025 9:40 AM, Xin Li wrote:
>>> +
>>>         handled = !handle_fastpath_set_x2apic_icr_irqoff(vcpu, data);
>>>         break;
>>>     case MSR_IA32_TSC_DEADLINE:
>>> -        data = kvm_read_edx_eax(vcpu);
>>> +        if (reg == VCPU_EXREG_EDX_EAX)
>>> +            data = kvm_read_edx_eax(vcpu);
>>> +        else
>>> +            data = kvm_register_read(vcpu, reg);
>>> +
>>
>> Hoist this chunk out of the switch clause to avoid duplication.
> 
> I thought about it, but didn't do so because the original code doesn't 
> read the MSR data from registers when a MSR is not being handled in the
> fast path, which saves some cycles in most cases.

I think I can make it an inline function.