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: <546391DE.7040508@citrix.com>
Date:	Wed, 12 Nov 2014 16:59:10 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	<x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <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 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?

> Hence I'd suggest
> 
> u64
> xen_swiotlb_get_required_mask(struct device *dev)
> {
> 	unsigned long max_mfn;
> 
> 	max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);
> 
> 	return DMA_BIT_MASK(__fls(max_mfn - 1) + 1 + PAGE_SHIFT);
> }
> 
> Jan
> 

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