Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall

From: Sohil Mehta <sohil.mehta@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <christian@brauner.io>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Shuah Khan <shuah@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Jonathan Corbet <corbet@lwn.net>, Raj Ashok <ashok.raj@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Gayatri Kammela <gayatri.kammela@intel.com>,
	Zeng Guang <guang.zeng@intel.com>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	Randy E Witt <randy.e.witt@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	Ramesh Thomas <ramesh.thomas@intel.com>,
	Linux API <linux-api@vger.kernel.org>,
	<linux-arch@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	<linux-kselftest@vger.kernel.org>
Subject: Re: [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall
Date: Fri, 1 Oct 2021 16:00:44 -0700	[thread overview]
Message-ID: <410ccb21-49d8-4ec0-3f23-07e37c694bbe@intel.com> (raw)
In-Reply-To: <875yug4eos.ffs@tglx>

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