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From: Wei Xu <xuwei5@hisilicon.com>
To: Julien Grall <julien@xen.org>, Jan Beulich <jbeulich@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Linuxarm <linuxarm@huawei.com>,
	Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>,
	"Zengtao \(B\)" <prime.zeng@hisilicon.com>,
	xen-devel@lists.xenproject.org, Volodymyr_Babchuk@epam.com
Subject: Re: [Xen-devel] [PATCH] arm/acpi: Add __acpi_unmap_table function for ARM
Date: Wed, 22 Jan 2020 13:58:46 +0800	[thread overview]
Message-ID: <5E27E496.5080102@hisilicon.com> (raw)
In-Reply-To: <6a003f33-f5ff-c027-f09b-85b80c6accdb@xen.org>

Hi Julien,

On 2020/1/21 19:25, Julien Grall wrote:
> Hi Jan,
> 
> On 21/01/2020 11:02, Jan Beulich wrote:
>> On 21.01.2020 10:49, Wei Xu wrote:
>>> Add __acpi_unmap_table function for ARM and invoke it at acpi_os_unmap_memory
>>> to make sure the related fixmap has been cleared before using it for a
>>> different mapping.
>>
>> How can it possibly be that this is needed for Arm only?
> 
> Let me give some background (I will let Wei dealing with the rest of the patches). On Arm, I made the decision to forbid a mapping replacement in the page-tables code. This means that if you want to re-use the same fixmap, then you need to clear it first.
> 
> The reason for the requirement is quite simple. On Arm, you need to use a break-before-make sequence any time you replace a valid entry by another valid entry (there is a couple of case where it is not needed).
> 
> As the sequence name suggests it, we will have a small window where the virtual address will point to nothing. This may result to an abort if another CPU is accessing the address at the same time.
> 
> In the fixmap case below, this should never happen. But now imagine shattering a superpage...
> 
> So rather than trying to allow in some cases the modification of a mapping, we just forbid for everything but permission changes. This is much simpler to reason and a much saner interface.
> 
> Similarly, I think this is much saner to call have a clear_fixmap() for each set_fixmap().

Thanks to explain!

Best Regards,
Wei

> 
> Cheers,
> 


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  reply	other threads:[~2020-01-22  5:59 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-21  9:49 [Xen-devel] [PATCH] arm/acpi: Add __acpi_unmap_table function for ARM Wei Xu
2020-01-21 10:01 ` Alexandru Stefan ISAILA
2020-01-22  1:51   ` Wei Xu
2020-01-21 11:02 ` Jan Beulich
2020-01-21 11:25   ` Julien Grall
2020-01-22  5:58     ` Wei Xu [this message]
2020-01-22  5:57   ` Wei Xu
2020-01-22  8:24     ` Jan Beulich
2020-01-22  9:04       ` Wei Xu

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