linux-kernel - Re: [Xen-devel] [PATCH 3/3] x86/xen: use the maximum MFN to calculate the required DMA mask

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Message-Id: <5463A47A0200007800046F19@mail.emea.novell.com>
Date:	Wed, 12 Nov 2014 17:18:34 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>
Cc:	<x86@...nel.org>, "Thomas Gleixner" <tglx@...utronix.de>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	<xen-devel@...ts.xenproject.org>,
	"BorisOstrovsky" <boris.ostrovsky@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 3/3] x86/xen: use the maximum MFN to
 calculate the required DMA mask

>>> On 12.11.14 at 17:59, <david.vrabel@...rix.com> wrote:
> On 12/11/14 15:55, Jan Beulich wrote:
>>>>> On 12.11.14 at 16:25, <david.vrabel@...rix.com> wrote:
>>> +u64
>>> +xen_swiotlb_get_required_mask(struct device *dev)
>>> +{
>>> +	u64 max_mfn;
>>> +
>>> +	max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);
>>> +
>>> +	return DMA_BIT_MASK(fls64(max_mfn << PAGE_SHIFT) + 1);
>>> +}
>> 
>> The value the hypercall returns is exclusive and an unsigned long.
> 
> The docs in include/public/memory.h say:
> 
> "Returns the maximum machine frame number of mapped RAM in this system."
> 
> Which sounds inclusive to me...  Do we need a doc update here?

Possibly - the origin of this is Linux'es mis-named "max_page" I believe.
And in any event I'd expect you to base your code changes on reality,
not just on documentation...

Jan

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