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Message-ID: <53f936d4-c6e0-3e9b-4291-8f16cbb87779@citrix.com>
Date: Wed, 26 Jul 2017 15:31:27 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Andy Lutomirski <luto@...capital.net>
CC: Juergen Gross <jgross@...e.com>, X86 ML <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH v1] xen: get rid of paravirt op
adjust_exception_frame
On 26/07/17 15:09, Andy Lutomirski wrote:
> On Wed, Jul 26, 2017 at 7:01 AM, Andrew Cooper
> <andrew.cooper3@...rix.com> wrote:
>> On 26/07/17 14:48, Andy Lutomirski wrote:
>>>> /* Runs on exception stack */
>>>> -ENTRY(nmi)
>>>> - /*
>>>> - * Fix up the exception frame if we're on Xen.
>>>> - * PARAVIRT_ADJUST_EXCEPTION_FRAME is guaranteed to push at most
>>>> - * one value to the stack on native, so it may clobber the rdx
>>>> - * scratch slot, but it won't clobber any of the important
>>>> - * slots past it.
>>>> - *
>>>> - * Xen is a different story, because the Xen frame itself overlaps
>>>> - * the "NMI executing" variable.
>>>> - */
>>> I would keep this comment. The Xen frame really is in the way AFAICT.
>> (For reasons best explained by the original authors) there is only ever
>> a single stack which a PV guest registers with Xen, which functions
>> equivalently to tss.sp0. There is no support for stack switching via
>> task switch or IST.
>>
>> Therefore, nested NMIs won't clobber the top of this stack.
>>
> Does that mean that nested NMIs on Xen just nest normally without
> clobbering each other?
Yes.
> If so, that's neat, although I wonder how we
> don't get crashes due to this:
>
> /* Normal 64-bit system call target */
> ENTRY(xen_syscall_target)
> undo_xen_syscall
> <-- NMI right here
> jmp entry_SYSCALL_64_after_swapgs
> ENDPROC(xen_syscall_target)
I'm going to go out on a limb here and say "because no has hit that
condition yet" (or at least managed to diagnose such a crash).
PV domU's don't get given NMIs. PV dom0 might, depending on how Xen is
handling the NMI itself. On XenServer at least, dom0 never gets handed
an NMI.
I expect is a sufficiently rarely used path that noone has noticed if it
is indeed broken.
~Andrew
>
> I think it would be nice if Xen could instead enter the native syscall
> path a bit later like this:
>
> ENTRY(entry_SYSCALL_64)
> /*
> * Interrupts are off on entry.
> * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
> * it is too small to ever cause noticeable irq latency.
> */
> SWAPGS_UNSAFE_STACK
> /*
> * A hypervisor implementation might want to use a label
> * after the swapgs, so that it can do the swapgs
> * for the guest and jump here on syscall.
> */
> GLOBAL(entry_SYSCALL_64_after_swapgs)
>
> movq %rsp, PER_CPU_VAR(rsp_scratch)
> movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
>
> TRACE_IRQS_OFF
>
> /* Construct struct pt_regs on stack */
> pushq $__USER_DS /* pt_regs->ss */
> pushq PER_CPU_VAR(rsp_scratch) /* pt_regs->sp */
> pushq %r11 /* pt_regs->flags */
> pushq $__USER_CS /* pt_regs->cs */
> pushq %rcx /* pt_regs->ip */
>
> <-- Xen enters here
>
> then we wouldn't have all this funny business. And Xen could
> completely skip the nmi nesting code.
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