linux-kernel - Re: [PATCH v1 1/2] KVM: arm64: determine memory type from VMA

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Date: Sun, 10 Mar 2024 03:49:36 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: Catalin Marinas <catalin.marinas@....com>, Jason Gunthorpe
	<jgg@...dia.com>
CC: "maz@...nel.org" <maz@...nel.org>, "oliver.upton@...ux.dev"
	<oliver.upton@...ux.dev>, "will@...nel.org" <will@...nel.org>, 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>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/2] KVM: arm64: determine memory type from VMA

Bringing this to the fore.

>> Where does that leave us for this patch? We check the VM_MTE_ALLOWED
>> and check for ZONE_MOVABLE struct pages as one of the conditions for
>> NORMAL?
>
> I think we should keep it as simple as possible and, looking at it
> again, maybe even ignore vm_page_prot. Two questions though:
>
> 1. Does VM_IO imply vm_page_prot never having MT_NORMAL or
>    MT_NORMAL_TAGGED?
>
> 2. Do all I/O ranges (side-effects, non-RAM) mapped into a guest (and
>   which end up in user_mem_abort()) imply VM_IO?
>
> If yes to both, I think something like below would do:

How may we get the answer to these? It seems to be the logical behavior, but
how can we confirm?

While we discuss on that, I am considering to send out the next version of this
patch (incorporating the feedbacks on the thread) that is rebased to a commit
inclusive of the recently applied KVM patch series:

[KVM: arm64: Allow the VM to select DEVICE_* and NORMAL_NC for IO memory]
https://lore.kernel.org/all/20240224150546.368-1-ankita@nvidia.com/

>        mte_allowed = kvm_vma_mte_allowed(vma);
>        noncacheable = false;                           // or 'device' as in user_mem_abort()
>        ...
>        if (vma->flags & VM_IO)                         // replaces !pfn_is_map_memory()
>                noncacheable = true;
>        else if (!mte_allowed && kvm_has_mte())
>                noncaheable = true;
>        ...
>        if (noncacheable)
>                prot |= KVM_PGTABLE_PROT_DEVICE;        // or the new KVM_PGTABLE_PROT_NC
>
> mte_allowed would cover DAX mappings (and, who knows, some future DAX
> mapping may allow MTE and the driver explicitly set the flag). Anything
> else hot-plugged into ZONE_MOVABLE should have VM_MTE_ALLOWED set or
> MTE disabled altogether.

--
Catalin