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Subject: Re: [PATCH v7 2/4] mm: introduce new flag to indicate wc safe
Thread-Topic: [PATCH v7 2/4] mm: introduce new flag to indicate wc safe
Thread-Index: AQHaXRJrmN1T1MCUwUqcUxbrSRtzmbEGsEcAgADxJG4=
Date: Tue, 13 Feb 2024 03:41:47 +0000
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References: <20240211174705.31992-1-ankita@nvidia.com>
 <20240211174705.31992-3-ankita@nvidia.com>
 <165363ba-d6cc-47a7-ab2a-d3a27a42f739@redhat.com>
In-Reply-To: <165363ba-d6cc-47a7-ab2a-d3a27a42f739@redhat.com>
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>> insufficient information and uncertainity in the behavior of=0A=
>=0A=
> s/uncertainity/uncertainty/=0A=
=0A=
Ack.=0A=
=0A=
>> non PCI drivers.=0A=
>>=0A=
>> Add a new flag VM_ALLOW_ANY_UNCACHED to indicate KVM that the device=0A=
>> is WC capable and these S2 changes can be extended to it. KVM can use=0A=
>> this flag to activate the code.=0A=
>>=0A=
>=0A=
> MM people will stumble only over this commit at some point, looking for=
=0A=
> details. It might make sense to add a bit more details on the underlying=
=0A=
> problem (user space tables vs. stage-1 vs. stage-2) and why we want to=0A=
> have a different mapping in user space compared to stage-1.=0A=
>=0A=
> Then, describe that the VMA flag was found to be the simplest and=0A=
> cleanest way to communicate this information from VFIO to KVM.=0A=
=0A=
Okay, I'll work on the commit message and describe in more details in=0A=
the next version.=0A=
=0A=
>> +/*=0A=
>> + * This flag is used to connect VFIO to arch specific KVM code. It=0A=
>> + * indicates that the memory under this VMA is safe for use with any=0A=
>> + * non-cachable memory type inside KVM. Some VFIO devices, on some=0A=
>> + * platforms, are thought to be unsafe and can cause machine crashes=0A=
>> + * if KVM does not lock down the memory type.=0A=
>> + */=0A=
>> +#ifdef CONFIG_64BIT=0A=
>> +#define VM_ALLOW_ANY_UNCACHED_BIT=A0=A0=A0 39=0A=
>> +#define VM_ALLOW_ANY_UNCACHED=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0 BIT(VM_ALLOW_ANY_UNCACHED_BIT)=0A=
>> +#else=0A=
>> +#define VM_ALLOW_ANY_UNCACHED=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0 VM_NONE=0A=
>> +#endif=0A=
>> +=0A=
>>=A0=A0 /* Bits set in the VMA until the stack is in its final location */=
=0A=
>>=A0=A0 #define VM_STACK_INCOMPLETE_SETUP (VM_RAND_READ | VM_SEQ_READ | VM=
_STACK_EARLY)=0A=
>>=0A=
>=0A=
> It's not perfect (very VFIO <-> KVM specific right now, VMA flags feel a=
=0A=
> bit wrong), but it certainly easier and cleaner than any alternatives I=
=0A=
> could think of.=0A=
>=0A=
> Acked-by: David Hildenbrand <david@redhat.com>=0A=
=0A=
Thanks! FWIW, it was found the cleanest way to restrict the changes to vfio=
-pci.=