linux-kernel - Re: [PATCH v1 3/4] KVM: VMX: Handle the immediate form of MSR instructions

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Message-ID: <f20842af-2bc1-4002-a6eb-84c33408d0ea@zytor.com>
Date: Thu, 31 Jul 2025 09:53:57 -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 3/4] KVM: VMX: Handle the immediate form of MSR
 instructions

On 7/31/2025 4:04 AM, Chao Gao wrote:
> On Wed, Jul 30, 2025 at 10:46:04AM -0700, Xin Li (Intel) wrote:
>> Handle two newly introduced VM exit reasons associated with the
>> immediate form of MSR instructions.
>>
>> For proper virtualization of the immediate form of MSR instructions,
>> Intel VMX architecture adds the following changes:
> 
> The CPUID feature bit also indicates support for the two new VM-exit reasons.
> Therefore, KVM needs to reflect EXIT_REASON_MSR_READ/WRITE_IMM VM-exits to
> L1 guests in nested cases if KVM claims it supports the new form of MSR
> instructions.

Damn, forgot about nested...

> 
> I'm also wondering if the emulator needs to support this new instruction. I
> suppose it does.

Yes, I thought about it.  However the new instructions use the VEX
prefix, which KVM doesn't support today.