From: Andrew Cooper <andrew.cooper3@citrix.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: George.Dunlap@eu.citrix.com, jbeulich@suse.com, julien@xen.org,
roger.pau@citrix.com, sstabellini@kernel.org, wl@xen.org,
xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH] x86/sm{e, a}p: do not enable SMEP/SMAP in PV shim by default on AMD
Date: Thu, 12 Oct 2023 16:33:05 +0800 [thread overview]
Message-ID: <a055111e-c04e-4975-9fdc-81d6e74f1c74@citrix.com> (raw)
In-Reply-To: <26fd7f099ab39ffccc2a5db8b4224fac6806461e.camel@infradead.org>
On 12/10/2023 4:21 pm, David Woodhouse wrote:
> On Thu, 2023-10-12 at 10:13 +0800, andrew.cooper3@citrix.com wrote:
>> On 11/10/2023 7:34 pm, David Woodhouse wrote:
>>> But why does the shim even need to turn it off when switching to the
>>> guest context? Its guest isn't running in supervisor mode so surely it
>>> doesn't *matter* whether SMEP is enabled or not? Why not just leave it
>>> on at all times?
>>
>> 32bit PV kernels run in Ring1. Which is supervisor and not user.
>
> Ah, thanks.
>
>> Some older PV kernels do execute on user pages, and don't like getting
>> SMEP faults when they didn't turn it on to begin with.
>
> PV guests never actually had the option to turn SMEP on, did they?
>
> (Otherwise I may have to rethink the approach of just putting
> 'smep=off' onto the shim command line when running under KVM...)
Xen and PV guests share a set of pagetables. There is no ability to
independently control SMEP/SMAP.
While we could in principle make SMEP an feature that PV kernels can opt
into, SMAP we really can't. The emulation costs of STAC/CLAC are
obscene from a perf perspective.
One TODO which has yet to be done is to look at the PV kernel's
ELF32/64-ness. For a shimmed 64bit guest, SMEP/SMAP should be on and
stay on.
TBH, it's probably best to just hide the SMEP/SMAP features, rather than
to play around with the cmdline.
~Andrew
next prev parent reply other threads:[~2023-10-12 8:33 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-16 16:00 [Xen-devel] [PATCH] x86/sm{e, a}p: do not enable SMEP/SMAP in PV shim by default on AMD Igor Druzhinin
2020-01-16 16:16 ` Andrew Cooper
2023-10-11 11:34 ` David Woodhouse
2023-10-12 2:13 ` andrew.cooper3
2023-10-12 8:21 ` David Woodhouse
2023-10-12 8:33 ` Andrew Cooper [this message]
2020-01-17 11:10 ` Jan Beulich
2020-01-20 14:07 ` Roger Pau Monné
2020-01-20 14:38 ` Jan Beulich
2020-01-20 15:37 ` Roger Pau Monné
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