linux-kernel - Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Message-ID:
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Date: Mon, 21 Apr 2025 16:03:54 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: Sean Christopherson <seanjc@...gle.com>, Jason Gunthorpe <jgg@...dia.com>
CC: Marc Zyngier <maz@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
	Oliver Upton <oliver.upton@...ux.dev>, "joey.gouly@....com"
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Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

> Hi, summarizing the discussion so far and outlining the next steps. The key points
> are as follows:
>
> 1. KVM cap to expose whether the kernel supports mapping cacheable PFNMAP:
> If the host doesn't have FWB, then the capability doesn't exist. Jason, Oliver, Caitlin
> and Sean points that this may not be required as userspace do not have
> much choice anyways. KVM has to follow the PTEs and userspace cannot ask
> for something different. However, Marc points that enumerating FWB support
> would allow userspace to discover the support and prevent live-migration
> across FWB and non-FWB hosts. Jason suggested that this may still be fine as
> we have already built in VFIO side protection where a live migration can be
> attempted and then fail because of late-detected HW incompatibilities.
>
> 2. New memslot flag that VMM passes at memslot registration:
> Discussion point that this is not necessary and KVM should just follow the
> VMA pgprot.
>
> 3. Fallback path handling for PFNMAP when the FWB is not set:
> Discussion points that there shouldn't be any fallback path and the memslot
> should just fail. i.e. KVM should not allow degrading cachable to non-cachable
> when it can't do flushing. This is to prevent the potential security issue
> pointed by Jason (S1 cacheable, S2 noncacheable).
>
> So AIU, the next step is to send out the updated series with the following patches:
> 1. Block cacheable PFN map in memslot creation (kvm_arch_prepare_memory_region)
> and during fault handling (user_mem_abort()).
>
> 2. Enable support for cacheable PFN maps if S2FWB is enabled by following
> the vma pgprot (this patch).
>
> 3. Add and expose the new KVM cap to expose cacheable PFNMAP (set to false
> for !FWB), pending maintainers' feedback on the necessity of this capability.

Hi, just a humble reminder to take a look at the summary and next steps.

Marc, will you be able to confirm if you still think we should have the
"cacheable PFNMAP" KVM cap?

If yes, I'll send out the series inclusive of that.

> Please let me know if there are any inaccuracies.
>
> Thanks
> Ankit Agrawal