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

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Message-ID: <bbb361bd-6cb8-7b9d-f689-d52bb1a05605@loongson.cn>
Date: Thu, 20 Mar 2025 11:30:43 +0800
From: bibo mao <maobibo@...ngson.cn>
To: Oliver Upton <oliver.upton@...ux.dev>,
 David Hildenbrand <david@...hat.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
 Jason Gunthorpe <jgg@...dia.com>, Marc Zyngier <maz@...nel.org>,
 Ankit Agrawal <ankita@...dia.com>, "joey.gouly@....com"
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Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags



On 2025/3/19 上午3:40, Oliver Upton wrote:
> On Tue, Mar 18, 2025 at 08:35:38PM +0100, David Hildenbrand wrote:
>> On 18.03.25 20:27, Catalin Marinas wrote:
>>> On Tue, Mar 18, 2025 at 09:55:27AM -0300, Jason Gunthorpe wrote:
>>>> On Tue, Mar 18, 2025 at 09:39:30AM +0000, Marc Zyngier wrote:
>>>>> The memslot must also be created with a new flag ((2c) in the taxonomy
>>>>> above) that carries the "Please map VM_PFNMAP VMAs as cacheable". This
>>>>> flag is only allowed if (1) is valid.
>>>>>
>>>>> This results in the following behaviours:
>>>>>
>>>>> - If the VMM creates the memslot with the cacheable attribute without
>>>>>     (1) being advertised, we fail.
>>>>>
>>>>> - If the VMM creates the memslot without the cacheable attribute, we
>>>>>     map as NC, as it is today.
>>>>
>>>> Is that OK though?
>>>>
>>>> Now we have the MM page tables mapping this memory as cachable but KVM
>>>> and the guest is accessing it as non-cached.
>>>
>>> I don't think we should allow this.
>>>
>>>> I thought ARM tried hard to avoid creating such mismatches? This is
>>>> why the pgprot flags were used to drive this, not an opt-in flag. To
>>>> prevent userspace from forcing a mismatch.
>>>
>>> We have the vma->vm_page_prot when the memslot is added, so we could use
>>> this instead of additional KVM flags.
>>
>> I thought we try to avoid messing with the VMA when adding memslots; because
>> KVM_CAP_SYNC_MMU allows user space for changing the VMAs afterwards without
>> changing the memslot?
> 
> Any checks on the VMA at memslot creation is done out of courtesy to
> userspace so it 'fails fast'. We repeat checks on the VMA at the time of
> fault to handle userspace twiddling VMAs behind our back.
yes, I think it is better to add cachable attribute in memslot, it can 
be checked on the VMA at memslot creation. Also cache attribute can be 
abstracted with cachable/uc/wc type rather than detailed arch specified.

Regards
Bibo Mao
> 
> VM_MTE_ALLOWED is an example of this.
> 
> Thanks,
> Oliver
>