linux-kernel - Re: [PATCH] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive

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Message-ID: <ade934de-0335-d2fb-11c1-5ecf05c800b2@linux.vnet.ibm.com>
Date:	Tue, 3 May 2016 15:36:05 +0800
From:	Yongji Xie <xyjxie@...ux.vnet.ibm.com>
To:	"Tian, Kevin" <kevin.tian@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Cc:	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"aik@...abs.ru" <aik@...abs.ru>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"mpe@...erman.id.au" <mpe@...erman.id.au>,
	"corbet@....net" <corbet@....net>,
	"warrier@...ux.vnet.ibm.com" <warrier@...ux.vnet.ibm.com>,
	"zhong@...ux.vnet.ibm.com" <zhong@...ux.vnet.ibm.com>,
	"nikunj@...ux.vnet.ibm.com" <nikunj@...ux.vnet.ibm.com>,
	"gwshan@...ux.vnet.ibm.com" <gwshan@...ux.vnet.ibm.com>
Subject: Re: [PATCH] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio
 page is exclusive

On 2016/5/3 14:11, Tian, Kevin wrote:

>> From: Yongji Xie [mailto:xyjxie@...ux.vnet.ibm.com]
>> Sent: Tuesday, May 03, 2016 1:52 PM
>>
>>>> +
>>>> +		if (!(res->start & ~PAGE_MASK)) {
>>>> +			/*
>>>> +			 * Add shadow resource for sub-page bar whose mmio
>>>> +			 * page is exclusive in case that hot-add device's
>>>> +			 * bar is assigned into the mem hole.
>>>> +			 */
>>>> +			shadow_res = kzalloc(sizeof(*shadow_res), GFP_KERNEL);
>>>> +			shadow_res->resource.start = res->end + 1;
>>>> +			shadow_res->resource.end = res->start + PAGE_SIZE - 1;
>>> What about res->start not page aligned so you end up still having
>>> a portion before res->start not exclusively reserved?
>> Do you mean add a 'dummy' resource to reserve the portion
>> before res->start if res->start not page aligned?
>>
>> But would it happen that there is a mem hole in the portion
>> before res->start? The resource should have been assigned
>> into the hole at the beginning.
>>
> Just a quick thought. Another device might occupy that range
> before initializing this device, and then 'another device' is hot
> removed later...
>
> Thanks
> Kevin

That's a good point! I will add support for this case in v2.

Thanks,
Yongji