linux-kernel - Re: [PATCH v7 2/4] mm: introduce new flag to indicate wc safe

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Date: Tue, 13 Feb 2024 03:41:47 +0000
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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.