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Date: Sun, 18 Jul 2010 14:03:54 +0300 From: Avi Kivity <avi@...hat.com> To: Linus Torvalds <torvalds@...ux-foundation.org> CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, LKML <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, Steven Rostedt <rostedt@...tedt.homelinux.com>, Frederic Weisbecker <fweisbec@...il.com>, Thomas Gleixner <tglx@...utronix.de>, Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>, Lai Jiangshan <laijs@...fujitsu.com>, Johannes Berg <johannes.berg@...el.com>, Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>, Arnaldo Carvalho de Melo <acme@...radead.org>, Tom Zanussi <tzanussi@...il.com>, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>, Andi Kleen <andi@...stfloor.org>, "H. Peter Anvin" <hpa@...or.com>, Jeremy Fitzhardinge <jeremy@...p.org>, "Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com> Subject: Re: [patch 1/2] x86_64 page fault NMI-safe On 07/15/2010 04:23 AM, Linus Torvalds wrote: > On Wed, Jul 14, 2010 at 3:37 PM, Linus Torvalds > <torvalds@...ux-foundation.org> wrote: > >> I think the %rip check should be pretty simple - exactly because there >> is only a single point where the race is open between that 'mov' and >> the 'iret'. So it's simpler than the (similar) thing we do for >> debug/nmi stack fixup for sysenter that has to check a range. >> > So this is what I think it might look like, with the %rip in place. > And I changed the "nmi_stack_ptr" thing to have both the pointer and a > flag - because it turns out that in the single-instruction race case, > we actually want the old pointer. > > Totally untested, of course. But _something_ like this might work: > > # > # Two per-cpu variables: a "are we nested" flag (one byte), and > # a "if we're nested, what is the %rsp for the nested case". > # > # The reason for why we can't just clear the saved-rsp field and > # use that as the flag is that we actually want to know the saved > # rsp for the special case of having a nested NMI happen on the > # final iret of the unnested case. > # > nmi: > cmpb $0,%__percpu_seg:nmi_stack_nesting > jne nmi_nested_corrupt_and_return > cmpq $nmi_iret_address,0(%rsp) > je nmi_might_be_nested > # create new stack > is_unnested_nmi: > # Save some space for nested NMI's. The exception itself > # will never use more space, but it might use less (since > # if will be a kernel-kernel transition). But the nested > # exception will want two save registers and a place to > # save the original CS that it will corrupt > subq $64,%rsp > > # copy the five words of stack info. 96 = 64 + stack > # offset of ss. > pushq 96(%rsp) # ss > pushq 96(%rsp) # rsp > pushq 96(%rsp) # eflags > pushq 96(%rsp) # cs > pushq 96(%rsp) # rip > > # and set the nesting flags > movq %rsp,%__percpu_seg:nmi_stack_ptr > movb $0xff,%__percpu_seg:nmi_stack_nesting > > By trading off some memory, we don't need this trickery. We can allocate two nmi stacks, so the code becomes: nmi: cmpb $0, %__percpu_seg:nmi_stack_nesting je unnested_nmi cmpq $nmi_iret,(%rsp) jne unnested_nmi cmpw $__KERNEL_CS,8(%rsp) jne unnested_nmi popf retfq unnested_nmi: xorq $(nmi_stack_1 ^ nmi_stack_2),%__percpu_seg:tss_nmi_ist_entry movb $1, __percpu_seg:nmi_stack_nesting regular_nmi: ... regular_nmi_end: movb $0, __percpu_seg:nmi_stack_nesting nmi_iret: iretq -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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