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Re: [Xen-devel] [PATCH] x86-64/Xen: fix stack switching

On Mon, May 7, 2018 at 5:16 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:

> While on native entry into the kernel happens on the trampoline stack,
> PV Xen kernels are being entered with the current thread stack right
> away. Hence source and destination stacks are identical in that case,
> and special care is needed.

> Other than in sync_regs() the copying done on the INT80 path as well as
> on the NMI path itself isn't NMI / #MC safe, as either of these events
> occurring in the middle of the stack copying would clobber data on the
> (source) stack. (Of course, in the NMI case only #MC could break
> things.)

I think I'd rather fix this by changing the stack switch code or
alternativing around it on non-stack-switching kernels.  Or make Xen use a
trampoline stack just like native.

> I'm not altering the similar code in interrupt_entry(), as that code
> path is unreachable when running an PV Xen guest afaict.

> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: stable@xxxxxxxxxx
> ---
> There would certainly have been the option of using alternatives
> patching, but afaict the patching code isn't NMI / #MC safe, so I'd
> rather stay away from patching the NMI path. And I thought it would be
> better to use similar code in both cases.

I would hope we do the patching before we enable any NMIs.

> Another option would be to make the Xen case match the native one, by
> going through the trampoline stack, but to me this would look like extra
> overhead for no gain.

Avoiding even more complexity in the nmi code seems like a big gain to me.

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