linux-kernel - Re: [Xen-devel] [PATCH 4/4] x86/xen: use the maximum MFN to calculate the required DMA mask

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Message-ID: <546DD3A8.2060404@citrix.com>
Date:	Thu, 20 Nov 2014 11:42:32 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	<linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<x86@...nel.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 4/4] x86/xen: use the maximum MFN to calculate
 the required DMA mask

On 19/11/14 17:51, Stefano Stabellini wrote:
> On Wed, 19 Nov 2014, David Vrabel wrote:
>> 
>> +u64
>> +xen_swiotlb_get_required_mask(struct device *dev)
>> +{
>> +	unsigned long max_mfn;
>> +
>> +	max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);
> 
> As Jan pointed out, I think you need to change the prototype of
> HYPERVISOR_memory_op to return long. Please do consistently across all
> relevant archs.

This doesn't help since 32-bit guests will still truncate.  A new
hypercall op that returns the result in a uint64_t parameter is required.

There is another reason why max_mfn isn't suitable -- IOMMU usage so I
think we should assume a 64-bit DMA mask is required (this is actually
the change I put into XenServer's kernel).

David
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