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

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Message-ID: <ZpGEQRJppKfp1tF8@google.com>
Date: Fri, 12 Jul 2024 12:30:09 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin3 Li <xin3.li@...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>, 
	"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>, 
	Ravi V Shankar <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

On Fri, Jul 12, 2024, Xin3 Li wrote:
> > > > E.g. if it's somewhere in task_struct, then kvm_on_user_return()
> > > > would restore the current task's desired RSP0.
> > >
> > > So you're suggesting to extend the framework to allow per task constants?
> > 
> > Yeah, or more likely, special case MSR_IA32_FRED_RSP0.  If KVM didn't already
> > have the user return framework, I wouldn't suggest this as I doubt avoiding
> > WRMSR when switching between vCPU tasks will be very meaningful, but it's
> > easy to handle FRED_RSP0, so why not.
> 
> Great, I will take the patch.
> 
> It looks to me that this also works for KERNEL GS BASE MSR, no?

I don't think so, because the kernel expects MSR_KERNEL_GS_BASE to be accurate
when querying GS.base for the current task:

  unsigned long x86_gsbase_read_task(struct task_struct *task)
  {
	unsigned long gsbase;

	if (task == current)
		gsbase = x86_gsbase_read_cpu_inactive();
	else if (boot_cpu_has(X86_FEATURE_FSGSBASE) ||
		 (task->thread.gsindex == 0))
		gsbase = task->thread.gsbase;
	else
		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);

	return gsbase;
  }