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From: Palmer Dabbelt <palmer@sifive.com>
To: aurelien@aurel32.net
Cc: linux-riscv@lists.infradead.org
Subject: Re: BUG: FP registers leak across execve
Date: Tue, 22 Jan 2019 12:31:16 -0800 (PST)	[thread overview]
Message-ID: <mhng-ee7d6384-45ad-440b-aa59-aaf56d8dfb12@palmer-si-x1c4> (raw)
In-Reply-To: <20190104013932.ksexuhjssygj4jml@aurel32.net>

On Thu, 03 Jan 2019 17:39:32 PST (-0800), aurelien@aurel32.net wrote:
> On 2019-01-03 15:36, Palmer Dabbelt wrote:
>> On Mon, 10 Dec 2018 14:13:28 PST (-0800), aurelien@aurel32.net wrote:
>> > Hi all,
>> >
>> > Debugging some glibc testsuite math failures, I have found out that most
>> > of the time, the FP status register and the FP registers are not zeroed
>> > as they should. This can be tested with the attached code. The best way
>> > to reproduce it is to execute from Python (i guess Perl or another
>> > interpreted language that support FP computation should work). When
>> > running an FP computation before calling the program, the result of the
>> > computation can be seen in f10.
>> >
>> > The zeroing of the FP status happens in kernel/process.c in the
>> > flush_thread function. It seems that the kernel restore that state only
>> > if a context switch happens between flush_thread and the first FP
>> > instruction of the executed program.
>> >
>> > A possible workaround is to restore of the FP registers in flush_thread,
>> > but that's probably not the best way to do that:
>> >
>> >
>> > --- a/arch/riscv/kernel/process.c
>> > +++ b/arch/riscv/kernel/process.c
>> > @@ -93,6 +93,7 @@ void flush_thread(void)
>> >          *      fflags: accrued exceptions cleared
>> >          */
>> >         memset(&current->thread.fstate, 0, sizeof(current->thread.fstate));
>> > +       fstate_restore(current, task_pt_regs(current));
>> >  #endif
>> >  }
>>
>> Are you running this in QEMU?  IIRC there was a bug here and we might not
>> have the right fix upstream yet.
>
> I can reproduce the issue in a QEMU 3.1 VM running a 4.20 kernel, but
> also on an HFU board running the original kernel.

OK, so at least it's not a QEMU bug.

I think this is the right place to put it, but we should also add

    regs->sstatus &= ~SR_FS_INITIAL;
    regs->sstatus |= SR_FS_INITIAL;

so the FPU's status register corresponds to the contents of the FPU.  I'm not 
sure if this is strictly necessary, but it'll certainly help FP save/restore 
performance.  I'm trying to construct a scenario where not setting setting 
SR.FS=initial actually manifests in incorrect behavior and can't think of one, 
but it does feel incorrect.

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      parent reply	other threads:[~2019-01-22 20:31 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-10 22:13 BUG: FP registers leak across execve Aurelien Jarno
2019-01-03 23:36 ` Palmer Dabbelt
2019-01-04  1:39   ` Aurelien Jarno
2019-01-04  3:12     ` Benjamin Herrenschmidt
2019-01-22 20:31     ` Palmer Dabbelt [this message]

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