Re: [PATCH] user-exec: Do not filter the signal on si_code
On 10/1/19 6:15 AM, Laurent Vivier wrote: > Le 01/10/2019 à 13:46, Peter Maydell a écrit : >> On Tue, 1 Oct 2019 at 12:19, Laurent Vivier wrote: >>> Is it possible to update the farm to Centos 8? >>> >>> Or as the kernel involved is specifically for POWER9, is it possible to >>> use only POWER8? >> >> My experience is that the gcc cfarm admins aren't in >> principle against the idea of upgrading farm machines, >> but in practice they tend to have a shortage of effort. >> If there's a centos-7-kernel-update package that could >> be installed without doing a full distro upgrade that >> would probably be pretty easy to ask them to arrange. > > It seems Centos provides a 4.18 kernel for POWER9 on Centos 7: > > http://mirror.centos.org/altarch/7/os/power9/Packages/kernel-4.18.0-80.7.2.el7.ppc64le.rpm Thanks guys. I've sent a message to the admins asking for an update on gcc135. r~
Re: [PATCH] user-exec: Do not filter the signal on si_code
Le 01/10/2019 à 13:46, Peter Maydell a écrit : > On Tue, 1 Oct 2019 at 12:19, Laurent Vivier wrote: >> Is it possible to update the farm to Centos 8? >> >> Or as the kernel involved is specifically for POWER9, is it possible to >> use only POWER8? > > My experience is that the gcc cfarm admins aren't in > principle against the idea of upgrading farm machines, > but in practice they tend to have a shortage of effort. > If there's a centos-7-kernel-update package that could > be installed without doing a full distro upgrade that > would probably be pretty easy to ask them to arrange. It seems Centos provides a 4.18 kernel for POWER9 on Centos 7: http://mirror.centos.org/altarch/7/os/power9/Packages/kernel-4.18.0-80.7.2.el7.ppc64le.rpm Thanks, Laurent
Re: [PATCH] user-exec: Do not filter the signal on si_code
On Tue, 1 Oct 2019 at 12:19, Laurent Vivier wrote: > Is it possible to update the farm to Centos 8? > > Or as the kernel involved is specifically for POWER9, is it possible to > use only POWER8? My experience is that the gcc cfarm admins aren't in principle against the idea of upgrading farm machines, but in practice they tend to have a shortage of effort. If there's a centos-7-kernel-update package that could be installed without doing a full distro upgrade that would probably be pretty easy to ask them to arrange. thanks -- PMM
Re: [PATCH] user-exec: Do not filter the signal on si_code
Le 01/10/2019 à 12:34, Peter Maydell a écrit : > On Mon, 30 Sep 2019 at 22:01, Richard Henderson > wrote: >> >> On 9/30/19 12:29 PM, Richard Henderson wrote: >>> This is a workaround for a ppc64le host kernel bug. > >>> However, the host kernel has supplied si_code == SEGV_MAPERR, >>> which is obviously incorrect. > >> It is disappointing about the kernel bug. But since this affects Centos 7, >> which is what *all* of the gcc compile farm ppc64 machines use, I think we >> need >> to work around it somehow. > > We knew about the ppc kernel bug in 2018: > https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg06049.html > and the decision at that time was to say "kernel bug, update your > kernel" :-) Is it possible to update the farm to Centos 8? Or as the kernel involved is specifically for POWER9, is it possible to use only POWER8? Thanks, Laurent
Re: [PATCH] user-exec: Do not filter the signal on si_code
On Mon, 30 Sep 2019 at 22:01, Richard Henderson wrote: > > On 9/30/19 12:29 PM, Richard Henderson wrote: > > This is a workaround for a ppc64le host kernel bug. > > However, the host kernel has supplied si_code == SEGV_MAPERR, > > which is obviously incorrect. > It is disappointing about the kernel bug. But since this affects Centos 7, > which is what *all* of the gcc compile farm ppc64 machines use, I think we > need > to work around it somehow. We knew about the ppc kernel bug in 2018: https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg06049.html and the decision at that time was to say "kernel bug, update your kernel" :-) thanks -- PMM
Re: [PATCH] user-exec: Do not filter the signal on si_code
On 9/30/19 12:29 PM, Richard Henderson wrote:
> This is a workaround for a ppc64le host kernel bug.
>
> For the test case linux-test, we have an instruction trace
>
> IN: sig_alarm
> ...
>
> IN:
> 0x400080ed28: 38ac li r0, 0xac
> 0x400080ed2c: 4402 sc
>
> IN: __libc_nanosleep
> 0x1003bb4c: 7c0802a6 mflr r0
> 0x1003bb50: f8010010 std r0, 0x10(r1)
>
> Our signal return trampoline has, rightly, changed the guest
> stack page read-only. Which, rightly, faults on the store of
> a return address into a stack frame.
>
> Checking the host /proc/pid/maps, we see the expected state:
>
> 400080-400081 r--p 00:00 0
>
> However, the host kernel has supplied si_code == SEGV_MAPERR,
> which is obviously incorrect.
>
> By dropping this check, we may have an extra walk of the page
> tables, but this should be inexpensive.
>
> Signed-off-by: Richard Henderson
> ---
>
> FWIW, filed as
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1757189
>
> out of habit and then
>
> https://bugs.centos.org/view.php?id=16499
>
> when I remembered that the system is running Centos not RHEL.
>
> ---
> accel/tcg/user-exec.c | 7 +--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c
> index 71c4bf6477..31ef091a70 100644
> --- a/accel/tcg/user-exec.c
> +++ b/accel/tcg/user-exec.c
> @@ -143,9 +143,12 @@ static inline int handle_cpu_signal(uintptr_t pc,
> siginfo_t *info,
> * for some other kind of fault that should really be passed to the
> * guest, we'd end up in an infinite loop of retrying the faulting
> * access.
> + *
> + * XXX: At least one host kernel, ppc64le w/Centos 7 4.14.0-115.6.1,
> + * incorrectly reports SEGV_MAPERR for a STDX write to a read-only page.
> + * Therefore, do not test info->si_code.
> */
> -if (is_write && info->si_signo == SIGSEGV && info->si_code ==
> SEGV_ACCERR &&
> -h2g_valid(address)) {
> +if (is_write && info->si_signo == SIGSEGV && h2g_valid(address)) {
Ho hum. This change is in conflict with Peter's long comment; I should have
read the context more thoroughly. There is an even longer comment with the
patch description: 9c4bbee9e3b83544257e82566342c29e15a88637
The SEGV_ACCERR check here is to prevent a loop by which page_unprotect races
with itself and, from Peter's analysis,
> * ...but when B gets the mmap lock it finds that the page is already
>PAGE_WRITE, and so it exits page_unprotect() via the "not due to
>protected translation" code path, and wrongly delivers the signal
>to the guest rather than just retrying the access
This bug was fixed in the referenced patch. But then continues:
> Since this would cause an infinite loop if we ever called
> page_unprotect() for some other kind of fault than "write failed due
> to bad access permissions", tighten the condition in
> handle_cpu_signal() to check the signal number and si_code, and add a
> comment so that if somebody does ever find themselves debugging an
> infinite loop of faults they have some clue about why.
>
> (The trick for identifying the correct setting for
> current_tb_invalidated for thread B (needed to handle the precise-SMC
> case) is due to Richard Henderson. Paolo Bonzini suggested just
> relying on si_code rather than trying anything more complicated.)
It is disappointing about the kernel bug. But since this affects Centos 7,
which is what *all* of the gcc compile farm ppc64 machines use, I think we need
to work around it somehow.
Should we simply add SEGV_MAPERR to the set of allowed si_code, to directly
work around the bug? If we got that code from a kernel without the bug, then
page_find should fail to find an entry, and we should then indicate that the
signal should be passed to the guest.
Thoughts?
r~
Re: [PATCH] user-exec: Do not filter the signal on si_code
Patchew URL: https://patchew.org/QEMU/20190930192931.20509-1-richard.hender...@linaro.org/ Hi, This series failed the asan build test. Please find the testing commands and their output below. If you have Docker installed, you can probably reproduce it locally. === TEST SCRIPT BEGIN === #!/bin/bash export ARCH=x86_64 make docker-image-fedora V=1 NETWORK=1 time make docker-test-debug@fedora TARGET_LIST=x86_64-softmmu J=14 NETWORK=1 === TEST SCRIPT END === The full log is available at http://patchew.org/logs/20190930192931.20509-1-richard.hender...@linaro.org/testing.asan/?type=message. --- Email generated automatically by Patchew [https://patchew.org/]. Please send your feedback to patchew-de...@redhat.com
