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

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Message-ID: <SA1PR11MB67341A4D3E4D11DAE8AF6D2EA8A62@SA1PR11MB6734.namprd11.prod.outlook.com>
Date: Fri, 12 Jul 2024 16:15:07 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: Sean Christopherson <seanjc@...gle.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"
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Subject: RE: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and
 guest

> > > > 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.

I probably should've given more details about how FRED RSPs are used:
RSP0 is a bit of special because it's a per task constant pointing to
its kernel stack top, while other RSPs are per CPU constants.

> Alternatively, is the desired RSP0 value tracked anywhere other than the MSR?

Yes, It's simply "(unsigned long)task_stack_page(task) + THREAD_SIZE".

> 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?

>  Even if we don't get fancy, avoiding the RDMSR
> to get the current task's value would be nice.

TBH, I didn't know RDMSR is NOT a preferred approach.  But it does make
sense because it costs hundreds cycles to read from CR2.

And of course this can be easily changed to
"(unsigned long)task_stack_page(task) + THREAD_SIZE".