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

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Message-ID: <SA1PR11MB67348BD07CCCF8D52FCAC8FEA8A42@SA1PR11MB6734.namprd11.prod.outlook.com>
Date: Wed, 10 Jul 2024 15:51:03 +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

> 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.