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Re: [Xen-devel] [PATCH] x86/sm{e, a}p: do not enable SMEP/SMAP in PV shim by default on AMD


  • To: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 12 Oct 2023 16:33:05 +0800
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  • Cc: George.Dunlap@xxxxxxxxxxxxx, jbeulich@xxxxxxxx, julien@xxxxxxx, roger.pau@xxxxxxxxxx, sstabellini@xxxxxxxxxx, wl@xxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Thu, 12 Oct 2023 08:33:28 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 12/10/2023 4:21 pm, David Woodhouse wrote:
> On Thu, 2023-10-12 at 10:13 +0800, andrew.cooper3@xxxxxxxxxx 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



 


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