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Re: [PATCH v2 3/6] configure: add option to enable LTO

From: Daniele Buono
Subject: Re: [PATCH v2 3/6] configure: add option to enable LTO
Date: Tue, 27 Oct 2020 16:42:11 -0400
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.1

Ok, no problem. I can definitely disable the check on GCC.

Paolo, would you like me to disable checks on AR/linker for lto too?
If so, should I add some of this information on a document, perhaps
docs/devel/lto.rst, so it is written somewhere for future uses?


Btw, using lto with gcc I found another interesting warning here
(adding scsi maintainer so they can chip in on the solution):

In function 'scsi_disk_new_request_dump',
inlined from 'scsi_new_request' at ../qemu-cfi-v3/hw/scsi/scsi-disk.c:2588:9: ../qemu-cfi-v3/hw/scsi/scsi-disk.c:2562:17: warning: argument 1 value '18446744073709551612' exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
     line_buffer = g_malloc(len * 5 + 1);
../qemu-cfi-v3/hw/scsi/scsi-disk.c: In function 'scsi_new_request':
/usr/include/glib-2.0/glib/gmem.h:78:10: note: in a call to allocation function 'g_malloc' declared here gpointer g_malloc (gsize n_bytes) G_GNUC_MALLOC G_GNUC_ALLOC_SIZE(1);

This seems like a bug to me. len is a signed integer filled up by
scsi_cdb_length which can return -1 if it can't decode the command.
What would probably happen is that we try a g_malloc with something too
big and that would fail. However, scsi_disk_new_request_dump is used for
tracing and:

a) I believe an unknown command here is a possibility, and is
handled by the caller - scsi_new_request - that has the following:

    command = buf[0];
    ops = scsi_disk_reqops_dispatch[command];
    if (!ops) {
        ops = &scsi_disk_emulate_reqops;

so a termination here on the malloc is probably not desired.

b) In the tracing, we should probably print the content of the buffer
anyway, so that the unknown command can be debugged. However, I don't
know what size I should use here.
I'm thinking either 1, to print just the command header in the buffer,
or the max size of the buffer, which I am not sure how to get.

Ideas or you prefer having an initial patch and then discuss it there?

On 10/27/2020 11:17 AM, Daniel P. Berrangé wrote:
On Tue, Oct 27, 2020 at 10:57:14AM -0400, Daniele Buono wrote:
In terms of ar and linker, if you don't have the right mix it will just
stop at link time with an error.

In terms of using gcc the errors may be a bit more subtle, similar to
what Daniel mentioned. Succesfully compiling but then showing issues at
runtime or in the test suite.

I'm using ubuntu 18.04 and the stock compiler (based on gcc 7.5) issues
a bunch of warnings but compile succesfully with LTO.
However, the tcg binary for sparc64 is broken. System-wide emulation
stops in OpenFirmware with an exception. User emulation triggers a
segmentation fault in some of the test cases. If I compile QEMU with
--enable-debug the tests magically work.

I briefly tested with gcc-9 and that seemed to work ok, buy your mileage
may vary

This why we shouldn't artificially block use of LTO with GCC in
the configure script. It blocks completely legitimate usage of
LTO with GCC versions where it works.

The user can detect if their version of GCC is broken by running the
test suite during their build process, which is best practice already,
and actually testing the result.

On 10/26/2020 11:50 AM, Daniel P. Berrangé wrote:
On Mon, Oct 26, 2020 at 10:51:43AM +0100, Paolo Bonzini wrote:
On 23/10/20 22:06, Daniele Buono wrote:
This patch allows to compile QEMU with link-time optimization (LTO).
Compilation with LTO is handled directly by meson. This patch adds checks
in configure to make sure the toolchain supports LTO.

Currently, allow LTO only with clang, since I have found a couple of issues
with gcc-based LTO.

In case fuzzing is enabled, automatically switch to llvm's linker (lld).
The standard bfd linker has a bug where function wrapping (used by the fuzz*
targets) is used in conjunction with LTO.

Tested with all major versions of clang from 6 to 12

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>

What are the problems like if you have GCC or you ar/linker are not up
to the job?  I wouldn't mind omitting the tests since this has to be
enabled explicitly by the user.

We temporarily disabled LTO in Fedora rawhide due to GCC bugs causing
wierd test suite asserts. Those were pre-release versions of GCC/binutils
though. I've just tested again and LTO works correctly, so I've enabled
LTO once again.



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