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Date: Thu, 29 Feb 2024 16:31:09 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: Jason Gunthorpe <jgg@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
"shameerali.kolothum.thodi@...wei.com"
<shameerali.kolothum.thodi@...wei.com>, "kevin.tian@...el.com"
<kevin.tian@...el.com>, Aniket Agashe <aniketa@...dia.com>, Neo Jia
<cjia@...dia.com>, Kirti Wankhede <kwankhede@...dia.com>, "Tarun Gupta
(SW-GPU)" <targupta@...dia.com>, Vikram Sethi <vsethi@...dia.com>, Andy
Currid <acurrid@...dia.com>, Alistair Popple <apopple@...dia.com>, John
Hubbard <jhubbard@...dia.com>, Dan Williams <danw@...dia.com>, Rahul
Rameshbabu <rrameshbabu@...dia.com>, Zhi Wang <zhiw@...dia.com>, "Anuj
Aggarwal (SW-GPU)" <anuaggarwal@...dia.com>, Matt Ochs <mochs@...dia.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/1] vfio/nvgrace-gpu: Convey kvm that the device is wc
safe
>> > + /*
>> > + * The VM_ALLOW_ANY_UNCACHED VMA flag is implemented for ARM64,
>> > + * allowing KVM stage 2 device mapping attributes to use Normal-NC
>> > + * rather than DEVICE_nGnRE, which allows guest mappings
>> > + * supporting write-combining attributes (WC). This also
>> > + * unlocks memory-like operations such as unaligned accesses
>> > + * This setting suits the fake BARs as they are expected to
>> > + * demonstrate such properties within the guest.
>> > + *
>> > + * ARM does not architecturally guarantee this is safe, and indeed
>> > + * some MMIO regions like the GICv2 VCPU interface can trigger
>> > + * uncontained faults if Normal-NC is used. The nvgrace-gpu
>> > + * however is safe in that the platform guarantees that no
>> > + * action taken on the MMIO mapping can trigger an uncontained
>> > + * failure. Hence VM_ALLOW_ANY_UNCACHED is set in the VMA flags.
>> > + */
>> > + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED);
>> > +
>> > return 0;
>> > }
>> >
>>
>> The commit log sort of covers it, but this comment doesn't seem to
>> cover why we're setting an uncached attribute to the usemem region
>> which we're specifically mapping as coherent... did we end up giving
>> this flag a really poor name if it's being used here to allow unaligned
>> access? Thanks,
>
> Also, this is setting the vma flag *after* the call to
> remap_pfn_range(), which seems quite sketchy. Thanks,
I will move it to the block that Jason pointed.
> Alex
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