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From: Dave Hansen <dave.hansen@intel.com>
To: Andy Lutomirski <luto@kernel.org>, Fenghua Yu <fenghua.yu@intel.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Tony Luck <tony.luck@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Ravi V Shankar <ravi.v.shankar@intel.com>,
	Peter Zijlstra <peterz@infradead.org>, x86 <x86@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	amd-gfx <amd-gfx@lists.freedesktop.org>,
	Christoph Hellwig <hch@infradead.org>,
	iommu <iommu@lists.linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Jacob Jun Pan <jacob.jun.pan@intel.com>,
	H Peter Anvin <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	David Woodhouse <dwmw2@infradead.org>,
	Felix Kuehling <Felix.Kuehling@amd.com>
Subject: Re: [PATCH v6 12/12] x86/traps: Fix up invalid PASID
Date: Mon, 3 Aug 2020 08:03:51 -0700	[thread overview]
Message-ID: <f6d34d59-e6eb-ee9f-d247-8fb2f0e37549@intel.com> (raw)
In-Reply-To: <CALCETrXnO4oh+WyxtSM-j_pP4QgkSg24=y76OBEHxXxAfJtPhA@mail.gmail.com>

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.
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  parent reply	other threads:[~2020-08-03 15:03 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-13 23:47 [PATCH v6 00/12] x86: tag application address space for devices Fenghua Yu
2020-07-13 23:47 ` [PATCH v6 01/12] iommu: Change type of pasid to u32 Fenghua Yu
2020-07-14  2:45   ` Liu, Yi L
2020-07-14 13:54     ` Fenghua Yu
2020-07-14 13:56       ` Liu, Yi L
2020-07-22 14:03   ` Joerg Roedel
2020-07-22 17:21     ` Fenghua Yu
2020-07-13 23:47 ` [PATCH v6 02/12] iommu/vt-d: Change flags type to unsigned int in binding mm Fenghua Yu
2020-07-13 23:47 ` [PATCH v6 03/12] docs: x86: Add documentation for SVA (Shared Virtual Addressing) Fenghua Yu
2020-07-14  3:25   ` Liu, Yi L
2020-07-15 23:32     ` Fenghua Yu
2020-07-13 23:47 ` [PATCH v6 04/12] x86/cpufeatures: Enumerate ENQCMD and ENQCMDS instructions Fenghua Yu
2020-07-13 23:48 ` [PATCH v6 05/12] x86/fpu/xstate: Add supervisor PASID state for ENQCMD feature Fenghua Yu
2020-07-13 23:48 ` [PATCH v6 06/12] x86/msr-index: Define IA32_PASID MSR Fenghua Yu
2020-07-13 23:48 ` [PATCH v6 07/12] mm: Define pasid in mm Fenghua Yu
2020-07-13 23:48 ` [PATCH v6 08/12] fork: Clear PASID for new mm Fenghua Yu
2021-02-24 10:19   ` Jean-Philippe Brucker
2021-02-25 22:17     ` Fenghua Yu
2021-03-01 23:00       ` Jacob Pan
2021-03-02 10:43         ` Jean-Philippe Brucker
2020-07-13 23:48 ` [PATCH v6 09/12] x86/process: Clear PASID state for a newly forked/cloned thread Fenghua Yu
2020-08-01  1:44   ` Andy Lutomirski
2020-07-13 23:48 ` [PATCH v6 10/12] x86/mmu: Allocate/free PASID Fenghua Yu
2020-07-13 23:48 ` [PATCH v6 11/12] sched: Define and initialize a flag to identify valid PASID in the task Fenghua Yu
2020-07-13 23:48 ` [PATCH v6 12/12] x86/traps: Fix up invalid PASID Fenghua Yu
2020-07-31 23:34   ` Andy Lutomirski
2020-08-01  0:42     ` Fenghua Yu
2020-08-03 15:03     ` Dave Hansen [this message]
2020-08-03 15:12       ` Andy Lutomirski
2020-08-03 15:19         ` Raj, Ashok
2020-08-03 16:36         ` Dave Hansen
2020-08-03 17:16           ` Andy Lutomirski
2020-08-03 17:34             ` Dave Hansen
2020-08-03 19:24               ` Andy Lutomirski
2020-08-01  1:28   ` Andy Lutomirski
2020-08-03 17:19     ` Fenghua Yu

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