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

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Message-ID: <ZpFH86n_YY5ModwK@google.com>
Date: Fri, 12 Jul 2024 08:12:51 -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 Wed, Jul 10, 2024, Xin3 Li wrote:
> > On Wed, Feb 07, 2024, Xin Li wrote:
> > > Switch MSR_IA32_FRED_RSP0 between host and guest in
> > > vmx_prepare_switch_to_{host,guest}().
> > >
> > > MSR_IA32_FRED_RSP0 is used during ring 3 event delivery only, thus
> > > KVM, running on ring 0, can run safely with guest FRED RSP0, i.e., no
> > > need to switch between host/guest FRED RSP0 during VM entry and exit.
> > >
> > > KVM should switch to host FRED RSP0 before returning to user level,
> > > and switch to guest FRED RSP0 before entering guest mode.
> > 
> > Heh, if only KVM had a framework that was specifically designed for context
> > switching MSRs on return to userspace.  Translation: please use the
> > user_return_msr() APIs.
> 
> IIUC the user return MSR framework works for MSRs that are per CPU
> constants, but like MSR_KERNEL_GS_BASE, MSR_IA32_FRED_RSP0 is a per
> *task* constant, thus we can't use it.

Ah, in that case, the changelog is very misleading and needs to be fixed.
Alternatively, is the desired RSP0 value tracked anywhere other than the MSR?
E.g. if it's somewhere in task_struct, then kvm_on_user_return() would restore
the current task's desired RSP0.  Even if we don't get fancy, avoiding the RDMSR
to get the current task's value would be nice.