linux-kernel - RE: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and guest

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Date: Fri, 19 Apr 2024 16:37:24 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: "Gao, Chao" <chao.gao@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
	<linux-kselftest@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "corbet@....net"
	<corbet@....net>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org"
	<x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>, "shuah@...nel.org"
	<shuah@...nel.org>, "vkuznets@...hat.com" <vkuznets@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>, "Shankar, Ravi V"
	<ravi.v.shankar@...el.com>, "xin@...or.com" <xin@...or.com>
Subject: RE: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and
 guest

> >+		if (unlikely(vmx->msr_host_fred_rsp0 == 0))
> >+			vmx->msr_host_fred_rsp0 =
> read_msr(MSR_IA32_FRED_RSP0);
> 
> can we just drop this and use "(unsigned long)task_stack_page(current) +
> THREAD_SIZE"
> as host fred rsp0?

I thought about it, however, don't see a strong reason that it's better,
 i.e., is RDMSR slower than reading 'stack' from current task_struct?

> 
> >+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);
> 
> any reason to not use wrmsrns?

Good call!


> >+	}
> > #else
> > 	savesegment(fs, fs_sel);
> > 	savesegment(gs, gs_sel);
> >@@ -1381,6 +1391,11 @@ static void vmx_prepare_switch_to_host(struct
> vcpu_vmx *vmx)
> > 	invalidate_tss_limit();
> > #ifdef CONFIG_X86_64
> > 	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
> >+
> >+	if (guest_can_use(&vmx->vcpu, X86_FEATURE_FRED)) {
> >+		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
> >+		wrmsrl(MSR_IA32_FRED_RSP0, vmx->msr_host_fred_rsp0);
> 
> same question.

Will do!

Thanks!
    Xin