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From: Arianna Avanzini <avanzini.arianna@gmail.com>
To: Ian Campbell <Ian.Campbell@eu.citrix.com>
Cc: paolo.valente@unimore.it, keir@xen.org,
	stefano.stabellini@eu.citrix.com, Ian.Jackson@eu.citrix.com,
	dario.faggioli@citrix.com, tim@xen.org, xen-devel@lists.xen.org,
	julien.grall@citrix.com, etrudeau@broadcom.com,
	JBeulich@suse.com, viktor.kleinik@globallogic.com
Subject: Re: [PATCH v5 3/8] arch, arm: let map_mmio_regions() take pfn as parameters
Date: Sat, 12 Apr 2014 11:20:46 +0200	[thread overview]
Message-ID: <5349056E.7050008@gmail.com> (raw)
In-Reply-To: <1397051683.6275.71.camel@kazak.uk.xensource.com>

Hello,

thank you for your comments, and sorry for the huge delay in replying.

On 04/09/2014 03:54 PM, Ian Campbell wrote:
> On Mon, 2014-04-07 at 01:31 +0200, Arianna Avanzini wrote:
>> -        res = map_mmio_regions(d, addr & PAGE_MASK,
>> -                               PAGE_ALIGN(addr + size) - 1,
>> -                               addr & PAGE_MASK);
>> +        res = map_mmio_regions(d,
>> +                               paddr_to_pfn(addr & PAGE_MASK),
>> +                               paddr_to_pfn_aligned(addr + size - 1),
> 
> 
> With
> +#define paddr_to_pfn_aligned(paddr)    paddr_to_pfn(PAGE_ALIGN(paddr))
> 
> There is a subtle difference here, which is that the "- 1" is now inside
> the align. Does this always have the same result? I'm not sure.
> 
> If addr+size == 0x1000 (page aligned) then:
> 
>         PAGE_ALIGN(0x10000)-1 = 0x10000-1 = 0xffff
> 
> But
> 
>         paddr_to_pfn_aligned(0x10000 - 1) =
>         paddr_to_pfn(PAGE_ALIGN(0xffff)) = paddr_to_pfn(0x10000) = 0x10
> 
> The new map_mmio_regions uses pfn_to_paddr which is:
> #define pfn_to_paddr(pfn) ((paddr_t)(pfn) << PAGE_SHIFT)
> 
> So with the old code the end address would be 0xffff, while with the new
> code it is 0x10<<12 = 0x10000.
> 
> I suspect the implementation of apply_to_p2m_changes is such that it
> doesn't actually change anything. Can you confirm that this was your
> intention?
> 

It wasn't my intention to change the size of the address range to be mapped,
thank you for pointing that out. As far as I can understand, the changes in this
patch would let apply_p2m_changes() use one extra address for the mapping; sorry
for that.

While preparing the patch I saw that, e.g., with addr + size = 0x10000,

end_gfn = paddr_to_pfn(PAGE_ALIGN(0x10000) - 1) =
          paddr_to_pfn(PAGE_ALIGN(0x10000)) - 1 = 0xf

and

pfn_to_paddr(end_gfn) = 0xf000

which seemed to me to be wrong, as the previous end address was, as you wrote,
0xffff.

Instead, if

end_gfn = paddr_to_pfn(PAGE_ALIGN(0x10000 - 1)) = 0x10

then

pfn_to_paddr(end_gfn) = 0x10000

which I thought lets the needed address range be mapped; however I didn't see
what you pointed out, that it also lets one extra address be used for the mapping.

> Is the end argument tio map_mmio_regions now intended to be inclusive or
> exclusive? 

As far as I understand, apply_p2m_changes(), which is called by the ARM version
of map_mmio_regions(), seems to take it as exclusive, as the mapping is
performed while (addr < end_gpaddr).
In the x86 version of map_mmio_regions() it is instead intended to be inclusive;
sorry for this mismatch, I'll try to make the behavior of the two versions
consistent.

> This sort of issue can be avoided by using a count instead of
> an end in the interface.
> 

Thank you very much for the hint.


> Ian.
> 



-- 
/*
 * Arianna Avanzini
 * avanzini.arianna@gmail.com
 * 73628@studenti.unimore.it
 */

  reply	other threads:[~2014-04-12  9:20 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-04-06 23:31 [PATCH v5 0/8] Implement the XEN_DOMCTL_memory_mapping hypercall for ARM Arianna Avanzini
2014-04-06 23:31 ` [PATCH v5 1/8] arch, arm: domain build: let dom0 access I/O memory of mapped devices Arianna Avanzini
2014-04-06 23:31 ` [PATCH v5 2/8] arch, arm: add consistency check to REMOVE p2m changes Arianna Avanzini
2014-04-07 11:05   ` Julien Grall
2014-04-09 13:39     ` Ian Campbell
2014-04-22 19:27       ` Julien Grall
2014-04-06 23:31 ` [PATCH v5 3/8] arch, arm: let map_mmio_regions() take pfn as parameters Arianna Avanzini
2014-04-07 14:56   ` Julien Grall
2014-04-09 13:54   ` Ian Campbell
2014-04-12  9:20     ` Arianna Avanzini [this message]
2014-04-06 23:31 ` [PATCH v5 4/8] arch, x86: check if mapping exists before memory_mapping removes it Arianna Avanzini
2014-04-06 23:31 ` [PATCH v5 5/8] xen, common: add the XEN_DOMCTL_memory_mapping hypercall Arianna Avanzini
2014-04-07  7:55   ` Jan Beulich
2014-04-09 14:03   ` Ian Campbell
2014-04-06 23:31 ` [PATCH v5 6/8] tools, libxl: parse optional start gfn from the iomem config option Arianna Avanzini
2014-04-09 14:10   ` Ian Campbell
2014-04-06 23:31 ` [PATCH v5 7/8] tools, libxl: add helpers to establish if guest is auto-translated Arianna Avanzini
2014-04-06 23:32 ` [PATCH v5 8/8] tools, libxl: handle the iomem parameter with the memory_mapping hcall Arianna Avanzini
2014-04-09 14:25   ` Ian Campbell

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