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

From: "Andy Lutomirski" <luto@kernel.org>
To: "Dave Hansen" <dave.hansen@intel.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Jon Kohler" <jon@nutanix.com>,
	"Sean Christopherson" <seanjc@google.com>
Cc: "Babu Moger" <babu.moger@amd.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Jim Mattson" <jmattson@google.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: Wed, 19 May 2021 16:15:16 -0700	[thread overview]
Message-ID: <b2a9056a-d181-44ae-bb9f-a809f52cf691@www.fastmail.com> (raw)
In-Reply-To: <8e45611c-f6ce-763a-ad17-adada33716d6@intel.com>

On Wed, May 19, 2021, at 3:44 PM, Dave Hansen wrote:
> On 5/17/21 12:46 AM, Paolo Bonzini wrote:
> > On 14/05/21 07:11, Andy Lutomirski wrote:
> >> That's nice, but it fails to restore XINUSE[PKRU].  As far as I know,
> >> that bit is live, and the only way to restore it to 0 is with
> >> XRSTOR(S).
> > 
> > The manual says "It is possible for XINUSE[i] to be 1 even when state
> > component i is in its initial configuration" so this is architecturally
> > valid.  Does the XINUSE optimization matter for PKRU which is a single
> > word?
> 
> In Linux with normal userspace, virtually never.
> 
> The hardware defaults PKRU to 0x0 which means "no restrictions on any
> keys".  Linux defaults PKRU via 'init_pkru_value' to the most
> restrictive value.  This ensures that new non-zero-pkey-assigned memory
> is protected by default.
> 
> But, that also means PKRU is virtually never in its init state in Linux.
>  An app would probably need to manipulate PKRU with XRSTOR to get
> XINUSE[PKRU]=0.
> 
> It would only even *possibly* be useful if running a KVM guest that had
> PKRU=0x0 (sorry I don't consider things using KVM "normal userspace" :P ).
> 

There was at least one report from the rr camp of glibc behaving differently depending on the result of XGETBV(1).  It's at least impolite to change the XINUSE register for a guest behind its back.

Admittedly that particular report wasn't about PKRU, and *Linux* guests won't run with XINUSE[PKRU]=0 under normal circumstances, but non-Linux guests could certainly do so.