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Date: Tue, 13 Feb 2024 03:41:47 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: David Hildenbrand <david@...hat.com>, Jason Gunthorpe <jgg@...dia.com>,
	"maz@...nel.org" <maz@...nel.org>, "oliver.upton@...ux.dev"
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	<bhe@...hat.com>
CC: 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>,
	"kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>, Matt Ochs
	<mochs@...dia.com>, Zhi Wang <zhiw@...dia.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 2/4] mm: introduce new flag to indicate wc safe

>> insufficient information and uncertainity in the behavior of
>
> s/uncertainity/uncertainty/

Ack.

>> non PCI drivers.
>>
>> Add a new flag VM_ALLOW_ANY_UNCACHED to indicate KVM that the device
>> is WC capable and these S2 changes can be extended to it. KVM can use
>> this flag to activate the code.
>>
>
> MM people will stumble only over this commit at some point, looking for
> details. It might make sense to add a bit more details on the underlying
> problem (user space tables vs. stage-1 vs. stage-2) and why we want to
> have a different mapping in user space compared to stage-1.
>
> Then, describe that the VMA flag was found to be the simplest and
> cleanest way to communicate this information from VFIO to KVM.

Okay, I'll work on the commit message and describe in more details in
the next version.

>> +/*
>> + * This flag is used to connect VFIO to arch specific KVM code. It
>> + * indicates that the memory under this VMA is safe for use with any
>> + * non-cachable memory type inside KVM. Some VFIO devices, on some
>> + * platforms, are thought to be unsafe and can cause machine crashes
>> + * if KVM does not lock down the memory type.
>> + */
>> +#ifdef CONFIG_64BIT
>> +#define VM_ALLOW_ANY_UNCACHED_BIT    39
>> +#define VM_ALLOW_ANY_UNCACHED                BIT(VM_ALLOW_ANY_UNCACHED_BIT)
>> +#else
>> +#define VM_ALLOW_ANY_UNCACHED                VM_NONE
>> +#endif
>> +
>>   /* Bits set in the VMA until the stack is in its final location */
>>   #define VM_STACK_INCOMPLETE_SETUP (VM_RAND_READ | VM_SEQ_READ | VM_STACK_EARLY)
>>
>
> It's not perfect (very VFIO <-> KVM specific right now, VMA flags feel a
> bit wrong), but it certainly easier and cleaner than any alternatives I
> could think of.
>
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks! FWIW, it was found the cleanest way to restrict the changes to vfio-pci.

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