linux-kernel - Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr

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Date:   Fri, 1 Oct 2021 16:00:44 -0700
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>
CC:     Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Raj Ashok <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        Linux API <linux-api@...r.kernel.org>,
        <linux-arch@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall

On 10/1/2021 2:29 PM, Thomas Gleixner wrote:
> So we'd end up with two XSAVES on context switch. We can simply do:
>          XSAVES();
>          fpu.state.xtsate.uintr.uinv = 0;

I am a bit confused. Do we need to set UINV to 0 explicitly?

If XSAVES gets called twice during context switch then the UINV in the 
XSTATE buffer automatically gets set to 0. Since XSAVES saves the 
current UINV value in the MISC_MSR which was already set to 0 by the 
previous XSAVES.

Though, this probably happens due to pure luck than intentional design :)

> which allows to do as many XRSTORS in a row as we want. Only the final
> one on the way to user space will have to restore the real vector if the
> register state is not valid:
>
>         if (fpu_state_valid()) {
>              if (needs_uinv(current)
>                 wrmsrl(UINV, vector);
>         } else {
>              if (needs_uinv(current)
>                 fpu.state.xtsate.uintr.uinv = vector;
>              XRSTORS();
>         }

I might have missed some subtle difference. Has this logic changed from 
what you previously suggested for arch_exit_to_user_mode_prepare()?

        if (xrstors_pending)) {
             // Update the saved xstate for xrstors
             // Unconditionally update the UINV since it could have been 
overwritten by calling XSAVES twice.
                current->xstate.uintr.uinv = UINTR_NOTIFICATION_VECTOR;
                 current->xstate.uintr.uirr |= pir;
         } else {
                 // Manually restore UIRR and UINV
                 rdmsrl(IA32_UINTR_RR, uirr);
                 wrmsrl(IA32_UINTR_RR, uirr | pir);

             misc.val64 = 0;
                 misc.uittsz = current->uintr->uittsz;
                 misc.uinv = UINTR_NOTIFICATION_VECTOR;
                 wrmsrl(IA32_UINTR_MISC, misc.val64);
         }

> Hmm?

The one case I can see this failing is if there was another XRSTORS 
after the "final" restore in arch_exit_to_user_mode_prepare()? I think 
that is not possible but I am not an expert on this. Did I misunderstand 
something?

Thanks,
Sohil