Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state

From: Jim Mattson <jmattson@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>, Jon Kohler <jon@nutanix.com>,
	Babu Moger <babu.moger@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Joerg Roedel <joro@8bytes.org>, Fenghua Yu <fenghua.yu@intel.com>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>,
	Tony Luck <tony.luck@intel.com>, Uros Bizjak <ubizjak@gmail.com>,
	Petteri Aimonen <jpa@git.mail.kapsi.fi>,
	Kan Liang <kan.liang@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@kernel.org>,
	Benjamin Thiel <b.thiel@posteo.de>,
	Fan Yang <Fan_Yang@sjtu.edu.cn>, Juergen Gross <jgross@suse.com>,
	Dave Jiang <dave.jiang@intel.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state
Date: Mon, 17 May 2021 11:15:07 -0700	[thread overview]
Message-ID: <CALMp9eS80a+Oy6spKT3cG7DCTW6jVwhyBuZ_t0SND=80Lg1XWA@mail.gmail.com> (raw)
In-Reply-To: <YKKwSLnkzc77HcnG@google.com>

On Mon, May 17, 2021 at 11:05 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, May 17, 2021, Dave Hansen wrote:
> > On 5/17/21 10:49 AM, Andy Lutomirski wrote:
> > >> The least awful solution would be to have the NMI handler restore
> > >> the host's PKRU.  The NMI handler would need to save/restore the
> > >> register, a la CR2, but the whole thing could be optimized to run
> > >> if and only if the NMI lands in the window where the guest's PKRU
> > >> is loaded.
> > >
> > > Or set a flag causing nmi_uaccess_ok() to return false.
> >
> > Oh, that doesn't sound too bad.  The VMENTER/EXIT paths are also
> > essentially a context switch.
>
> I like that idea, too.
>
> The flag might also be useful to fix the issue where the NMI handler activates
> PEBS after KVM disables it.  Jim?

The issue is actually that the NMI handler *clears* IA32_PEBS_ENABLE
bits after giving out the host value of the MSR to KVM. If we were to
block the NMI handler from modifying IA32_PEBS_ENABLE until after the
next VM-exit, that could solve this issue. I don't know if it makes
sense to piggyback on nmi_uaccess(), though.

> > Will widening the window where nmi_uaccess_okay()==false anger any of
> > the perf folks?  It looks like perf knows how to handle it nicely.