Re: [Xen-devel] [PATCH] x86, paravirt, xen: Remove the 64-bit irq_enable_sysexit pvop

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/06/2015 04:03 PM, Andy Lutomirski wrote:
> On Mon, Apr 6, 2015 at 11:30 AM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 04/06/2015 01:44 PM, Andrew Cooper wrote:
> > > On 06/04/2015 16:29, Andy Lutomirski wrote:
> > > > On Mon, Apr 6, 2015 at 7:10 AM, Konrad Rzeszutek Wilk
> > > > <konrad.wilk@xxxxxxxxxx> wrote:
> > > > > On Fri, Apr 03, 2015 at 03:52:30PM -0700, Andy Lutomirski wrote:
> > > > > > [cc: Boris and Konrad.  Whoops]
> > > > > >
> > > > > > On Fri, Apr 3, 2015 at 3:51 PM, Andy Lutomirski <luto@xxxxxxxxxx>
> > > > > > wrote:
> > > > > > > We don't use irq_enable_sysexit on 64-bit kernels any more.  Remove
> > > > > Is there an commit (or name of patch) that explains why
> > > > > 32-bit-user-space-on-64-bit
> > > > > kernels is unsavory?
> > > > sysexit never tasted very good :-p
> > > >
> > > > We're (hopefully) not breaking 32-bit-user-space-on-64-bit, but we're
> > > > trying an unconventional approach to making the code faster and less
> > > > scary.  As a result, 64-bit kernels won't use sysexit any more.
> > > > Hopefully Xen is okay with the slightly sneaky thing we're doing.
> > > > AFAICT Xen thinks of sysretl and sysexit as slightly funny irets, so I
> > > > don't expect there to be any problem.
> > > 64bit PV kernels must bounce through Xen to switch from the kernel to
> > > the user pagetables (since both kernel and userspace are both actually
> > > running in ring3 with user pages).
> > >
> > > As a result, exit to userspace ends up as a hypercall into Xen which has
> > > an effect very similar to an `iret`, but with some extra fixup in the
> > > background.
> > >
> > > I can't forsee any Xen issues as a result of this patch.
> >
> > I ran tip plus this patch (plus another patch that fixes an unrelated Xen
> > regression in tip) through our test suite and it completed without problems.
> >
> > I also ran some very simple 32-bit programs in a 64-bit PV guest and didn't
> > see any problems there neither.
> At the risk of redundancy, did you test on Intel hardware?  At least
> on native systems, the code in question never executes on AMD systems.
Yes, the tests ran on Intel. I left them scheduled for overnight runs 
too and that will be executed on both AMD and Intel.
-boris

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