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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>,
	xen-devel <xen-devel@lists.xenproject.org>
Cc: 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 11:21:57 +0100
Message-ID: <eb44801a-1081-858d-4853-6dc574e0c6d6@citrix.com> (raw)
In-Reply-To: <5D074BA30200007800238B6C@prv1-mh.provo.novell.com>

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.

> (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.

~Andrew

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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 [this message]
2019-07-03 10:34       ` Jan Beulich
2019-07-03 19:47         ` Andrew Cooper
2019-07-04  9:09           ` Jan Beulich

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