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From: Shanker R Donthineni <sdonthineni@nvidia.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	<linux-arm-kernel@lists.infradead.org>,
	<kvmarm@lists.cs.columbia.edu>, <linux-kernel@vger.kernel.org>,
	<kvm@vger.kernel.org>, Vikram Sethi <vsethi@nvidia.com>,
	Jason Sequeira <jsequeira@nvidia.com>
Subject: Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA
Date: Fri, 30 Apr 2021 06:25:08 -0500
Message-ID: <e3d7fda8-5263-211c-3686-f699765ab715@nvidia.com> (raw)
In-Reply-To: <20210429134659.321a5c3c@redhat.com>

Hi Alex

On 4/29/21 2:46 PM, Alex Williamson wrote:
> If an alignment fault is fixed by configuring a WC mapping, doesn't
> that suggest that the driver performed an unaligned access itself and
> is relying on write combining by the processor to correct that error?
> That's wrong.  Fix the driver or please offer another explanation of
> how the WC mapping resolves this.  I suspect you could enable tracing
> in QEMU, disable MMIO mmaps on the vfio-pci device and find the invalid
> access.
>
>> We've two concerns here:
>>    - Performance impacts for pass-through devices.
>>    - The definition of ioremap_wc() function doesn't match the host
>> kernel on ARM64
> Performance I can understand, but I think you're also using it to mask
> a driver bug which should be resolved first.  Thank

We’ve already instrumented the driver code and found the code path for the unaligned
accesses. We’ll fix this issue if it’s not following WC semantics.

Fixing the performance concern will be under KVM stage-2 page-table control. We're
looking for a guidance/solution for updating stage-2  PTE based on PCI-BAR attribute.



  parent reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-29 16:29 [RFC 0/2] [RFC] Honor PCI prefetchable attributes for a virtual machine on ARM64 Shanker Donthineni
2021-04-29 16:29 ` [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA Shanker Donthineni
2021-04-29 18:28   ` Alex Williamson
2021-04-29 19:14     ` Shanker R Donthineni
2021-04-29 19:46       ` Alex Williamson
2021-04-29 22:08         ` Vikram Sethi
2021-04-30 11:25         ` Shanker R Donthineni [this message]
     [not found]           ` <87czucngdc.wl-maz@kernel.org>
2021-04-30 13:07             ` Shanker R Donthineni
2021-04-30 14:58             ` Shanker R Donthineni
     [not found]               ` <878s4zokll.wl-maz@kernel.org>
2021-04-30 16:57                 ` Vikram Sethi
2021-05-01  9:30                   ` Marc Zyngier
2021-05-01 11:36                     ` Shanker R Donthineni
     [not found]                       ` <87czu8uowe.wl-maz@kernel.org>
2021-05-03 12:08                         ` Shanker R Donthineni
2021-05-02 17:56                     ` Vikram Sethi
2021-05-03 10:17                       ` Marc Zyngier
2021-05-03 13:35                         ` Mark Kettenis
2021-05-03 13:59                           ` Vikram Sethi
2021-05-03 14:44                             ` Alex Williamson
2021-05-03 22:03                               ` Vikram Sethi
2021-05-04  8:30                                 ` Will Deacon
2021-05-05 18:02                                   ` Catalin Marinas
2021-05-06  7:22                                     ` Christoph Hellwig
2021-05-08 16:33                                     ` Shanker R Donthineni
2021-06-02  9:37                                       ` Marc Zyngier
2021-05-04 18:03                                 ` Alex Williamson
2021-06-02  9:11                                   ` Marc Zyngier
2021-04-30  9:54   ` Lorenzo Pieralisi
2021-04-30 12:38     ` Jason Gunthorpe
2021-04-29 16:29 ` [RFC 2/2] KVM: arm64: Add write-combine support for stage-2 entries Shanker Donthineni
2021-05-03  7:01 ` [RFC 0/2] [RFC] Honor PCI prefetchable attributes for a virtual machine on ARM64 Christoph Hellwig

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