From: Julien Grall <julien@xen.org>
To: Jan Beulich <jbeulich@suse.com>,
Oleksandr Tyshchenko <olekstysh@gmail.com>
Cc: "Oleksandr Tyshchenko" <oleksandr_tyshchenko@epam.com>,
"Stefano Stabellini" <sstabellini@kernel.org>,
"Bertrand Marquis" <bertrand.marquis@arm.com>,
"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
"Andrew Cooper" <andrew.cooper3@citrix.com>,
"George Dunlap" <george.dunlap@citrix.com>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
"Julien Grall" <jgrall@amazon.com>,
xen-devel@lists.xenproject.org, "Henry Wang" <Henry.Wang@arm.com>
Subject: Re: [PATCH V7 2/2] xen/gnttab: Store frame GFN in struct page_info on Arm
Date: Tue, 11 Oct 2022 14:33:29 +0100 [thread overview]
Message-ID: <d297735d-771e-22f2-1b46-9c390ebb5afd@xen.org> (raw)
In-Reply-To: <77349d2b-de72-3e68-362a-493f350eeec4@suse.com>
Hi Jan,
On 11/10/2022 14:28, Jan Beulich wrote:
> On 11.10.2022 15:01, Julien Grall wrote:
>> Hi Jan,
>>
>> On 11/10/2022 12:59, Jan Beulich wrote:
>>> On 16.07.2022 16:56, Oleksandr Tyshchenko wrote:
>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>>
>>>> Rework Arm implementation to store grant table frame GFN
>>>> in struct page_info directly instead of keeping it in
>>>> standalone status/shared arrays. This patch is based on
>>>> the assumption that a grant table page is a xenheap page.
>>>>
>>>> To cover 64-bit/40-bit IPA on Arm64/Arm32 we need the space
>>>> to hold 52-bit/28-bit + extra bit value respectively. In order
>>>> to not grow the size of struct page_info borrow the required
>>>> amount of bits from type_info's count portion which current
>>>> context won't suffer (currently only 1 bit is used on Arm).
>>>
>>> I'm afraid this isn't true: There's no requirement for a guest to pass
>>> all different GFNs to VCPUOP_register_vcpu_info, yet map_vcpu_info()
>>> tries to obtain a reference for every vCPU.
>>
>> AFAIU, this would be a reference of the **count_info** not **type_info**
>> (which BTW will never be incremented on Arm because we have no type
>> support).
>
> I should have said "obtain a writable type reference".
Thanks for the clarification.
>
>> The commit message is only referring to the 'type_info's count'. So...
>>
>>> With my adding of GFN
>>> (really gaddr) based registration of the runstate area (already
>>> looking towards 4.18) the maximum possible count is to further grow.
>>
>> ... I am not sure which problem you are referring too.
>
> Wow - a mere stub (but not inline) function to make the build happy.
> Then why is the description talking about one bit that's needed on
> Arm?
Because share_xen_page_with_guest() will always set the type info's
count to 1.
TBH I don't exactly know why we set it. I always assumed this was a
requirement for the common code but never checked.
Cheers,
--
Julien Grall
next prev parent reply other threads:[~2022-10-11 13:33 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-16 14:56 [PATCH V7 1/2] xen/arm: Harden the P2M code in p2m_remove_mapping() Oleksandr Tyshchenko
2022-07-16 14:56 ` [PATCH V7 2/2] xen/gnttab: Store frame GFN in struct page_info on Arm Oleksandr Tyshchenko
2022-07-16 15:08 ` Julien Grall
2022-07-16 15:20 ` Oleksandr Tyshchenko
2022-07-18 8:53 ` Jan Beulich
2022-10-11 11:59 ` Jan Beulich
2022-10-11 13:01 ` Julien Grall
2022-10-11 13:28 ` Jan Beulich
2022-10-11 13:33 ` Julien Grall [this message]
2022-10-11 13:38 ` Jan Beulich
2022-10-17 13:46 ` Jan Beulich
2022-10-17 14:37 ` Julien Grall
2022-07-16 15:06 ` [PATCH V7 1/2] xen/arm: Harden the P2M code in p2m_remove_mapping() Julien Grall
2022-07-16 15:29 ` Oleksandr Tyshchenko
2022-07-20 18:20 ` Julien Grall
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