Re: [PATCH v6 12/12] x86/traps: Fix up invalid PASID

[Dave Hansen]

On 7/31/20 4:34 PM, Andy Lutomirski wrote:
>> Thomas suggested to provide a reason for the #GP caused by executing ENQCMD
>> without a valid PASID value programmed. #GP error codes are 16 bits and all
>> 16 bits are taken. Refer to SDM Vol 3, Chapter 16.13 for details. The other
>> choice was to reflect the error code in an MSR. ENQCMD can also cause #GP
>> when loading from the source operand, so its not fully comprehending all
>> the reasons. Rather than special case the ENQCMD, in future Intel may
>> choose a different fault mechanism for such cases if recovery is needed on
>> #GP.
> Decoding the user instruction is ugly and sets a bad architecture
> precedent, but we already do it in #GP for UMIP.  So I'm unconvinced.

I'll try to do one more bit of convincing. :)

In the end, we need a way to figure out if the #GP was from a known "OK"
source that we can fix up.  You're right that we could fire up the
instruction decoder to help answer that question.  But, it (also)
doesn't easily yield a perfect answer as to the source of the #GP, it
always involves a user copy, and it's a larger code impact than what
we've got.

I think I went and looked at fixup_umip_exception(), and compared it to
the alternative which is essentially just these three lines of code:

> +	/*
> +	 * If the current task already has a valid PASID in the MSR,
> +	 * the #GP must be for some other reason.
> +	 */
> +	if (current->has_valid_pasid)
> +		return false;
...> +	/* Now the current task has a valid PASID in the MSR. */
> +	current->has_valid_pasid = 1;

and *I* was convinced that instruction decoding wasn't worth it.

There's a lot of stuff that fixup_umip_exception() does which we don't
have to duplicate, but it's going to be really hard to get it anywhere
near as compact as what we've got.

I guess Fenghua could go give instruction decoding a shot and see how it
looks, though.