Re: [PATCH 6/7] x86/traps: Fix up invalid PASID

From: Thomas Gleixner <tglx@linutronix.de>
To: "Raj\, Ashok" <ashok.raj@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Jacob Jun Pan <jacob.jun.pan@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Sohil Mehta <sohil.mehta@intel.com>,
	Ravi V Shankar <ravi.v.shankar@intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>,
	iommu@lists.linux-foundation.org, Ashok Raj <ashok.raj@intel.com>
Subject: Re: [PATCH 6/7] x86/traps: Fix up invalid PASID
Date: Tue, 28 Apr 2020 02:54:59 +0200	[thread overview]
Message-ID: <874kt4pgyk.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200427224646.GA103955@otc-nc-03>

Ashok,

"Raj, Ashok" <ashok.raj@intel.com> writes:
> On Sun, Apr 26, 2020 at 05:25:06PM +0200, Thomas Gleixner wrote:
>> Just for the record I also suggested to have a proper errorcode in the
>> #GP for ENQCMD and I surely did not suggest to avoid decoding the user
>> instructions.
>
> We certainly discussed the possiblity of adding an error code to 
> identiy #GP due to ENQCMD with our HW architects. 
>
> There are only a few cases that have an error code, like move to segment
> with an invalid value for instance. There were a few but i don't
> recall that entire list. 
>
> Since the error code is 0 in most places, there isn't plumbing in hw to return
> this value in all cases. It appeared that due to some uarch reasons it
> wasn't as simple as it appears to /me sw kinds :-)

Sigh.

> So after some internal discussion we decided to take the current
> approach. Its possible that if the #GP was due to some other reason
> we might #GP another time. Since this wasn't perf or speed path we took
> this lazy approach.

I know that the HW people's mantra is that everything can be fixed in
software and therefore slapping new features into the CPUs can be done
without thinking about the consequeses.

But we all know from painful experience that this is fundamentally wrong
unless there is a really compelling reason.

For new features there is absolutely no reason at all.

Can HW people pretty please understand that hardware and software have
to be co-designed and not dictated by 'some uarch reasons'. This is
nothing fundamentally new. This problem existed 30+ years ago, is well
documented and has been ignored forever. I'm tired of that, really.

But as this seems to be unsolvable for the problem at hand can you
please document the inability, unwillingness or whatever in the
changelog?

The question why this brand new_ ENQCMD + invalid PASID induced #GP does
not generate an useful error code and needs heuristics to be dealt with
is pretty obvious.

Thanks,

        tglx