linux-kernel - Re: [RFC PATCH 10/13] x86/uintr: Introduce user IPI sender syscalls

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Date:   Wed, 29 Sep 2021 09:04:54 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Sohil Mehta <sohil.mehta@...el.com>
Cc:     x86@...nel.org, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 10/13] x86/uintr: Introduce user IPI sender syscalls

On Tue, Sep 28, 2021 at 11:01:54AM -0700, Sohil Mehta wrote:
> On 9/23/2021 5:28 AM, Greg KH wrote:
> > On Mon, Sep 13, 2021 at 01:01:29PM -0700, Sohil Mehta wrote:
> > > +/* User Interrupt Target Table Entry (UITTE) */
> > > +struct uintr_uitt_entry {
> > > +	u8	valid;			/* bit 0: valid, bit 1-7: reserved */
> > Do you check that the other bits are set to 0?
> 
> I don't have a check but kzalloc() in alloc_uitt() should set it to 0.
> 
> > > +	u8	user_vec;
> > > +	u8	reserved[6];
> > What is this reserved for?
> 
> This is hardware defined structure as well. I should probably mention this
> it in the comment above.
> 
> > > +	u64	target_upid_addr;
> > If this is a pointer, why not say it is a pointer?
> 
> I used a u64 to get the size and alignment of this structure as required by
> the hardware. I wasn't sure if using a struct upid * would complicate that.
> 
> Also this field is never used as a pointer by the kernel. It is only used to
> program an entry that is read by the hardware.
> 
> Is this reasonable or would you still prefer a pointer?

Ok, just document it really well that this is NOT a real address used by
the kernel.  As it is, that's not obvious at all.

And if this crosses the user/kernel boundry, it needs to be __u64 right?

thanks,

greg k-h