From: Thomas Gleixner <tglx@linutronix.de>
To: "Jacob Pan \(Jun\)" <jacob.jun.pan@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>, Ashok Raj <ashok.raj@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, jacob.jun.pan@intel.com
Subject: Re: [PATCH 5/7] x86/mmu: Allocate/free PASID
Date: Tue, 28 Apr 2020 20:54:01 +0200 [thread overview]
Message-ID: <87tv13o306.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200428112113.000033bd@intel.com>
"Jacob Pan (Jun)" <jacob.jun.pan@intel.com> writes:
> On Sun, 26 Apr 2020 16:55:25 +0200
> Thomas Gleixner <tglx@linutronix.de> wrote:
>> Fenghua Yu <fenghua.yu@intel.com> writes:
>> > The PASID is freed when the process exits (so no need to keep
>> > reference counts on how many SVM devices are sharing the PASID).
>>
>> I'm not buying that. If there is an outstanding request with the PASID
>> of a process then tearing down the process address space and freeing
>> the PASID (which might be reused) is fundamentally broken.
>>
> Device driver unbind PASID is tied to FD release. So when a process
> exits, FD close causes driver to do the following:
>
> 1. stops DMA
> 2. unbind PASID (clears the PASID entry in IOMMU, flush all TLBs, drain
> in flight page requests)
Fair enough. Explaining that somewhere might be helpful.
> For bare metal SVM, if the last mmdrop always happens after FD release,
> we can ensure no outstanding requests at the point of ioasid_free().
> Perhaps this is a wrong assumption?
If fd release cleans up then how should there be something in flight at
the final mmdrop?
> For guest SVM, there will be more users of a PASID. I am also
> working on adding refcounting to ioasid. ioasid_free() will not release
> the PASID back to the pool until all references are dropped.
What does more users mean?
>> > + if (mm && mm->context.pasid && !(flags &
>> > SVM_FLAG_PRIVATE_PASID)) {
>> > + /*
>> > + * Once a PASID is allocated for this mm, the PASID
>> > + * stays with the mm until the mm is dropped. Reuse
>> > + * the PASID which has been already allocated for
>> > the
>> > + * mm instead of allocating a new one.
>> > + */
>> > + ioasid_set_data(mm->context.pasid, svm);
>>
>> So if the PASID is reused several times for different SVMs then every
>> time ioasid_data->private is set to a different SVM. How is that
>> supposed to work?
>>
> For the lifetime of the mm, there is only one PASID. svm_bind/unbind_mm
> could happen many times with different private data: intel_svm.
> Multiple devices can bind to the same PASID as well. But private data
> don't change within the first bind and last unbind.
Ok. I read through that spaghetti of intel_svm_bind_mm() again and now I
start to get an idea how that is supposed to work. What a mess.
That function really wants to be restructured in a way so it is
understandable to mere mortals.
Thanks,
tglx
next prev parent reply other threads:[~2020-04-28 18:54 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-30 19:33 [PATCH 0/7] x86: tag application address space for devices Fenghua Yu
2020-03-30 19:33 ` [PATCH 1/7] docs: x86: Add a documentation for ENQCMD Fenghua Yu
2020-04-26 11:02 ` Thomas Gleixner
2020-04-27 20:13 ` Fenghua Yu
2020-03-30 19:33 ` [PATCH 2/7] x86/cpufeatures: Enumerate ENQCMD and ENQCMDS instructions Fenghua Yu
2020-04-26 11:06 ` Thomas Gleixner
2020-04-27 20:17 ` Fenghua Yu
2020-03-30 19:33 ` [PATCH 3/7] x86/fpu/xstate: Add supervisor PASID state for ENQCMD feature Fenghua Yu
2020-04-26 11:17 ` Thomas Gleixner
2020-04-27 20:33 ` Fenghua Yu
2020-03-30 19:33 ` [PATCH 4/7] x86/msr-index: Define IA32_PASID MSR Fenghua Yu
2020-04-26 11:22 ` Thomas Gleixner
2020-04-27 20:50 ` Fenghua Yu
2020-03-30 19:33 ` [PATCH 5/7] x86/mmu: Allocate/free PASID Fenghua Yu
2020-04-26 14:55 ` Thomas Gleixner
2020-04-27 22:18 ` Fenghua Yu
2020-04-27 23:44 ` Thomas Gleixner
2020-04-28 18:21 ` Jacob Pan (Jun)
2020-04-28 18:54 ` Thomas Gleixner [this message]
2020-04-28 19:07 ` Luck, Tony
2020-04-28 20:42 ` Jacob Pan (Jun)
2020-04-28 20:59 ` Luck, Tony
2020-04-28 22:13 ` Jacob Pan (Jun)
2020-04-28 22:32 ` Luck, Tony
2020-04-28 20:40 ` Jacob Pan (Jun)
2020-04-28 20:57 ` Fenghua Yu
2020-03-30 19:33 ` [PATCH 6/7] x86/traps: Fix up invalid PASID Fenghua Yu
2020-04-26 15:25 ` Thomas Gleixner
2020-04-27 20:11 ` Fenghua Yu
2020-04-28 0:13 ` Thomas Gleixner
2020-04-27 22:46 ` Raj, Ashok
2020-04-27 23:08 ` Luck, Tony
2020-04-28 0:20 ` Thomas Gleixner
2020-04-28 0:54 ` Thomas Gleixner
2020-04-28 1:08 ` Raj, Ashok
2020-03-30 19:33 ` [PATCH 7/7] x86/process: Clear PASID state for a newly forked/cloned thread Fenghua Yu
2020-04-22 20:41 ` [PATCH 0/7] x86: tag application address space for devices Fenghua Yu
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