linux-kernel - Re: [RFC PATCH v5 1/2] selftests/x86: add xsave test during and after signal handling

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Date:   Mon, 13 Dec 2021 12:19:52 +0800
From:   Pengfei Xu <pengfei.xu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Shuah Khan <skhan@...uxfoundation.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Heng Su <heng.su@...el.com>, Luck Tony <tony.luck@...el.com>,
        Mehta Sohil <sohil.mehta@...el.com>,
        Chen Yu C <yu.c.chen@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH v5 1/2] selftests/x86: add xsave test during and
 after signal handling

Hi Dave,

On 2021-12-11 at 16:02:06 +0800, Pengfei Xu wrote:
> Hi Dave,
> 
> On 2021-12-10 at 08:48:08 -0800, Dave Hansen wrote:
> > On 12/9/21 8:47 PM, Pengfei Xu wrote:
> > > How about the following changes:
> > > Will remove set_avx2_ymm() and will only check XSAVE_MASK_FP, XSAVE_MASK_OPMASK
> > > and XSAVE_MASK_PKRU xstates after signal handling and process switch,
> > 
> > First and foremost, the whole point of these tests is to ensure that the
> > kernel is properly maintaining register state.  Removing registers from
> > the test moves *away* from the primary goal of this test.
> > 
>  Thanks for suggestion!
>  Actually, I already removed any useless libc function before and after
>  xsave action, only left the test action between xsave action:
> "
> 	XSAVE(xsave_buf2, XSAVE_TEST_MASK);
> 	do raise signal or fork test
> 	XSAVE(xsave_buf3, XSAVE_TEST_MASK);
> "
>   I found that after fork() function in virtual machine, XMM0 or XMM1 register
> will be used and changed.
>   But in YMM xstate, I haven't see signal handline and fork action will use
> and change YMM regiseters in the test. Seems we could keep YMM xstate test.
>   Seems it needs some other better way for XMM xstate.
> 
> > Second, you just listed three states there.  Have you considered looking
> > at whether those have the same problem as the XMM/YMM registers?  Please do.
> > 
>   I have tested FP, AVX512 opmask and pkru xstates on different platforms and
> virtual machine, gdb these 3 xstates with fork and signal handling even printf,
> above 3 functions will not use and change above 3 xstates. I used previous
> xsave instruction tests to get the results.
> 
> > Third (and I've also suggested this before), we should explicitly tell
> > the compiler not to use the FPU registers.  This is what the kernel
> > does, and it's what allows us to, for instance, make function calls in
> > the kernel without clobbering userspace content in XSAVE-managed registers.
> > 
> > If we did that, then we would only have to worry about calls to things
> > *outside* of the test program, like libc.
>   Thanks! Yes, I will add "float a = 0.12, b = 0.34; a = a + b;" to tell
> libc process, float points has been used.
>   Seems if there is no addition, subtraction, multiplication or division,
> there is no change in FP xstate compared with no float definition. If there
> is above operation, mxcsr(xstate offset 0x18-0x1b bytes)will change from 801f
> to a01f. Rounding control bit change from 00 to 01.
> 
  Sorry, I misunderstood your meaning in the last email, I should directly add
  syscall function between the 2 xsave comparisons, it will not use XMM, YMM
  and so on xstates, and I could add them back.
  For example, for "fork", I will use syscall(SYS_fork) instead of fork().

> Thanks!
> BR.