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 10:38:31 +0000
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <xen-devel@...ts.xenproject.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Boris Ostrovsky <boris.ostrovsky@...cle.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.

All hypercalls return long, despite lack of clarity, or in some cases,
documentation to the contrary.

Almost all hypercalls need the ability to return errors in the form of
negative numbers, and those which don't should not be treated any
differently.  Furthermore, the overflow conditions for the compat
version of XENMEM_maximum_ram_page specifically truncate to INT_MAX and
INT_MIN as boundaries, making the return value signed.

~Andrew

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