linux-kernel - Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

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Message-ID: <7c685ae6-938b-9d99-20e6-f51398543441@nvidia.com>
Date:   Fri, 30 Apr 2021 08:07:46 -0500
From:   Shanker R Donthineni <sdonthineni@...dia.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "Christoffer Dall" <christoffer.dall@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, Vikram Sethi <vsethi@...dia.com>,
        Jason Sequeira <jsequeira@...dia.com>
Subject: Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR
 region in VMA


Hi Marc,

On 4/30/21 6:47 AM, Marc Zyngi
>>>> 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.
> Before we start discussing the *how*, I'd like to clearly understand
> what *arm64* memory attributes you are relying on. We already have
> established that the unaligned access was a bug, which was the biggest
> argument in favour of NORMAL_NC. What are the other requirements?
>
We are  relying  on NORMAL_NC mapping for PCI prefetchable BARs instead of DEVICE_nGnRE in baremetal and VM.
No other requirement other than this.

-Shanker