linux-kernel - Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Message-ID: <20250114133145.GA5556@nvidia.com>
Date: Tue, 14 Jan 2025 09:31:45 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: David Hildenbrand <david@...hat.com>
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Subject: Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

On Tue, Jan 14, 2025 at 02:17:50PM +0100, David Hildenbrand wrote:

> I assume MTE does not apply at all to VM_PFNMAP, at least
> arch_calc_vm_flag_bits() tells me that VM_MTE_ALLOWED should never get set
> there.

As far as I know, it is completely platform specific what addresses MTE
will work on. For instance, I would expect a MTE capable platform with
CXL to want to make MTE work on the CXL memory too.

However, given we have no way of discovery, limiting MTE to boot time
system memory seems like the right thing to do for now - which we can
achieve by forbidding it from VM_PFNMAP.

Having VFIO add VM_MTE_ALLOWED someday might make sense if someone
solves the discoverability problem.

Jason