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

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Date:	Thu, 13 Nov 2014 09:57:38 +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>,
	"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 at 16:55, <JBeulich@...e.com> 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 latter has further relevance: For one, HYPERVISOR_memory_op()
returns "int" currently - this needs to become "long" for the purpose
here (also for eventual future uses of XENMEM_maximum_gpfn and
XENMEM_{current,maximum}_reservation).

And then, using u64 rather than unsigned long for max_mfn would
still cause problems for 32-bit kernels on systems with memory
extending beyond the 8Tb boundary (since the long -> u64
conversion really goes long -> s64 -> u64).

Jan

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