Xen-Devel Archive on lore.kernel.org
 help / color / Atom feed
From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>, WeiLiu <wl@xen.org>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v2 2/2] x86/traps: widen condition for logging top-of-stack
Date: Wed, 3 Jul 2019 20:47:18 +0100
Message-ID: <48d87646-5fa2-4cb7-e7a4-e83af1f372dd@citrix.com> (raw)
In-Reply-To: <2e2a1d5d-f04e-5400-6817-86a7985b1c9b@suse.com>

On 03/07/2019 11:34, Jan Beulich wrote:
> On 03.07.2019 12:21, Andrew Cooper wrote:
>> On 17/06/2019 09:13, Jan Beulich wrote:
>>> Despite -fno-omit-frame-pointer the compiler may omit the frame pointer,
>>> often for relatively simple leaf functions.
>> Actually, the problem is more widespread than this.  For every function,
>> there is a non-zero quantity of time between the function starting and
>> the frame pointer being set up.
>>
>> However, half of this time is spent with the old %rbp on the top of the
>> stack, so won't benefit from these changes.
> I think the compiler typically pairs push %rbp and mov %rsp, %rbp,
> but this pair may not sit at the beginning of the function. And it's
> that other code that's prone to crash. The push %rbp may also fault
> (most notably due to stack overrun), but that would then still have
> the top of stack covered by the change here. The mov %rsp, %rbp,
> otoh, won't plausibly fault. IOW I think it's far more than "half of
> the time" that this change helps.

My statement wasn't meant as a criticism, but more of an observation.

>
>>> (To give a specific example,
>>> the case I've run into this with is _pci_hide_device() and gcc 8.
>>> Interestingly the even more simple neighboring iommu_has_feature() does
>>> get a frame pointer set up, around just a single instruction. But this
>>> may be a result of the size-of-asm() effects discussed elsewhere.)
>>>
>>> Log the top-of-stack value if it looks valid _or_ if RIP looks invalid.
>> This far, I'm happy with.
>>
>>> Also annotate non-frame-pointer-based stack trace entries with a
>>> question mark, to signal clearly that any one of them may not actually
>>> be part of the call stack.
>> I'm still opposed to this.  The introduction of ? does more harm than
>> good IMO, because it simply can't be trusted.
>>
>> Stack traces are not guaranteed-accurate, even with frame pointers
>> enabled.  The only thing we can say for certain in any trace is where
>> %rip points.
> Yes, I realize you still don't like this. But similarly to the
> other patch set - on the v1 discussion here I was lacking
> feedback, and hence I eventually timed out and sent v2. The
> question is - what is your alternative proposal to distinguish
> the truly guessed entry logged here from the more reliable
> ones? And then similarly how to distinguish the less reliable
> ones produced by the !CONFIG_FRAME_POINTER variant of
> _show_trace() from their more reliable counterparts?

A crazy idea I've just had.  Annotate all printed lines with a character
identifying which source of information we used?

We could have [r] for register state, [f] for "from frame pointer", and
[s] for "from stack rubble".

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-31  8:59 [PATCH 0/2]: x86/traps: improve show_trace()'s top-of-stack handling Jan Beulich
2019-05-31  8:59 ` [Xen-devel] " Jan Beulich
2019-05-31  9:17 ` [PATCH 1/2] x86/traps: guard top-of-stack reads Jan Beulich
2019-05-31  9:17   ` [Xen-devel] " Jan Beulich
2019-06-07 17:51   ` Andrew Cooper
2019-06-11  9:57     ` Jan Beulich
2019-05-31  9:22 ` [PATCH 2/2] x86/traps: widen condition for logging top-of-stack Jan Beulich
2019-05-31  9:22   ` [Xen-devel] " Jan Beulich
2019-06-07 18:01   ` Andrew Cooper
2019-06-11  9:46     ` Jan Beulich
2019-06-17  8:10 ` [Xen-devel] [PATCH v2 0/2]: x86/traps: improve show_trace()'s top-of-stack handling Jan Beulich
2019-06-17  8:12   ` [Xen-devel] [PATCH v2 1/2] x86/traps: guard top-of-stack reads Jan Beulich
2019-07-02 17:47     ` Andrew Cooper
2019-07-03  7:10       ` Jan Beulich
2019-06-17  8:13   ` [Xen-devel] [PATCH v2 2/2] x86/traps: widen condition for logging top-of-stack Jan Beulich
2019-07-03 10:21     ` Andrew Cooper
2019-07-03 10:34       ` Jan Beulich
2019-07-03 19:47         ` Andrew Cooper [this message]
2019-07-04  9:09           ` Jan Beulich

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=48d87646-5fa2-4cb7-e7a4-e83af1f372dd@citrix.com \
    --to=andrew.cooper3@citrix.com \
    --cc=JBeulich@suse.com \
    --cc=roger.pau@citrix.com \
    --cc=wl@xen.org \
    --cc=xen-devel@lists.xenproject.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Xen-Devel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/xen-devel/0 xen-devel/git/0.git
	git clone --mirror https://lore.kernel.org/xen-devel/1 xen-devel/git/1.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 xen-devel xen-devel/ https://lore.kernel.org/xen-devel \
		xen-devel@lists.xenproject.org xen-devel@archiver.kernel.org
	public-inbox-index xen-devel


Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.xenproject.lists.xen-devel


AGPL code for this site: git clone https://public-inbox.org/ public-inbox