Re: [uml-devel] [PATCH 1/3] um: extend _fpstate to _xstate

[Richard Weinberger <richard@nod.at>]

Eli,

Am 12.03.2016 um 08:08 schrieb Eli Cooper:
> Hi Richard,
> 
> On 2016/3/10 4:44, Richard Weinberger wrote:
>> Hmm, this needs rework. Having everything on the stack is not good.
> 
> Okay, I'll rework the functions whose stack size is greater than the
> warning threshold by using kmalloc.

I fear it is not that easy. Having a kmalloc() per context switch would
be every expensive. Even for UML.

>> Can you also create a selftest such that this bug cannot happen again?
> 
> It seems that instead of writing a self-test showing this problem cannot
> happen again, I wrote a test that manifested another bug that is not
> directly related to my patch.
> 
> Without applying my patch, the current UML should support XMM registers
> because those are covered by _fpstate and PTRACE_GETFPREGS. But it
> seemed that XMM registers are not restored after the signal handler returns.
> 
> In the following quick test, the main loop should run indefinitely
> despite XMM registers are modified by the signal handler. But in UML,
> the loop breaks randomly within a minute or two, showing that the
> registers are corrupted. So far I haven't found the cause. Any hints?

Meh. :(
Can you figure out whether the issue depends on the host kernel? i.e. try something older
and Linus' tree.
UML is a heavy user of ptrace(), maybe the recent FPU cleanup on x86 broke something.

Thanks,
//richard

> Thanks,
> Eli
> 
> ---
> /* test if signal handling preserves XMM registers */
> #include <stdio.h>
> #include <unistd.h>
> #include <signal.h>
> 
> int count;
> 
> void sighandler(int signum)
> {
>         count++;
> 
>         /* alarm(1) without calling libc */
>         asm("mov $0x1,%rdi");
>         asm("mov $0x25,%rax");
>         asm("syscall");
> 
>         asm("movq $0xdeadbeef,%r11");
>         /* the following two instructions
>          * modify xmm0 and xmm1 registers */
>         asm("vmovq %r11,%xmm0");
>         asm("vmovq %r11,%xmm1");
> }
> 
> int main()
> {
>         struct sigaction act;
>         double a = 3.14159, b = 2.71828;
> 
>         act.sa_handler = sighandler;
>         act.sa_flags = 0;
>         sigemptyset(&act.sa_mask);
>         sigaction(SIGALRM, &act, NULL);
> 
>         alarm(1);
> 
>         /* this loop should run indefinitely */
>         while (a + b == a + b) ;
> 
>         printf("count = %d\n", count);
>         return 1;
> }
> 

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