linux-kernel - Re: [PATCH 13/15] arm64: kvm: Rewrite fake pgd handling

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Message-ID: <561D2B7C.5050005@arm.com>
Date:	Tue, 13 Oct 2015 17:04:12 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Catalin.Marinas@....com, Will.Deacon@....com, Mark.Rutland@....com,
	Marc.Zyngier@....com, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, ard.biesheuvel@...aro.org
Subject: Re: [PATCH 13/15] arm64: kvm: Rewrite fake pgd handling

On 13/10/15 16:39, Christoffer Dall wrote:
> On Mon, Oct 12, 2015 at 10:55:24AM +0100, Suzuki K. Poulose wrote:
>> On 10/10/15 15:52, Christoffer Dall wrote:
>>> Hi Suzuki,
>>
>> Hi Christoffer,
>>
>> Thanks for being patient enough to review the code :-) without much of
>> the comments. I now realise there needs much more documentation than
>> what I have put in already. I am taking care of this in the next
>> revision already.
>>
>>> I had to refresh my mind a fair bit to be able to review this, so I
>>> thought it may be useful to just remind us all what the constraints of
>>> this whole thing is, and make sure we agree on this:
>>>
>>> 1. We fix the IPA max width to 40 bits
>>> 2. We don't support systems with a PARange smaller than 40 bits (do we
>>>     check this anywhere or document this anywhere?)
>>
>> AFAIT, no we don't check it anywhere. May be we should. We could plug this
>> into my CPU feature infrastructure[1] and let the is_hype_mode_available()
>> use the info to decide if we can support 40bit IPA ?
>>
>
> If we support 40bit IPA or more, yes, I think that would be sane.  Or at
> least put a comment somewhere, perhaps in Documenation.

OK

>>> 3. We always assume we are running on a system with PARange of 40 bits
>>>     and we are therefore constrained to use concatination.
>>>
>>> As an implication of (3) above, this code will attempt to allocate 256K
>>> of physically contiguous memory for each VM on the system.  That is
>>> probably ok, but I just wanted to point it out in case it raises any
>>> eyebrows for other people following this thread.
>>
>> Right, I will document this in a comment.
>>
>>>> level:  0       1         2         3
>>>> bits : [47] [46 - 36] [35 - 25] [24 - 14] [13 - 0]
>>>>           ^       ^     ^
>>>>           |       |     |
>>>>     host entry    |     x---- stage-2 entry
>>>>                   |
>>>>          IPA -----x
>>>
>>> Isn't the stage-2 entry using bits [39:25], because you resolve
>>> more than 11 bits on the initial level of lookup when you concatenate
>>> tables?
>>
>> Yes, the stage-2 entry is just supposed to show the entry level (2).
>>
>
> I don't understand, the stage-2 entry level will be at bit 39, not 35?
>

That picture shows the 'level 2' at which the stage-2 translations begin,
with 16 pages concatenated, which gives 39-25. The host kernel macros,
normally only sees upto bit 35, which is fixed using the kvm_pgd_index()
to pick the right PGD entry for a VA.

Thanks
Suzuki

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