linux-kernel - Re: [PATCH v2 19/20] xen/privcmd: Add support for Linux 64KB page granularity

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Date:	Tue, 14 Jul 2015 00:05:44 +0200
From:	Julien Grall <julien.grall@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<linux-arm-kernel@...ts.infradead.org>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [PATCH v2 19/20] xen/privcmd: Add support for Linux 64KB page
 granularity

Hi Boris,

On 13/07/2015 22:13, Boris Ostrovsky wrote:
> On 07/09/2015 04:42 PM, Julien Grall wrote:
>> -
>>   struct remap_data {
>>       xen_pfn_t *fgmfn; /* foreign domain's gmfn */
>> +    xen_pfn_t *efgmfn; /* pointer to the end of the fgmfn array */
>
> It might be better to keep size of fgmfn array instead.

It would means to have an other variable to check that we are at the end 
the array.

What about a variable which will be decremented?

>>
>> +static int unmap_gfn(struct page *page, unsigned long pfn, void *data)
>> +{
>> +    int *nr = data;
>> +    struct xen_remove_from_physmap xrp;
>> +
>> +    /* The Linux Page may not have been fully mapped to Xen */
>> +    if (!*nr)
>> +        return 0;
>> +
>> +    xrp.domid = DOMID_SELF;
>> +    xrp.gpfn = pfn;
>> +    (void)HYPERVISOR_memory_op(XENMEM_remove_from_physmap, &xrp);
>> +
>> +    (*nr)--;
>> +
>> +    return 0;
>> +}
>> +
>>   int xen_xlate_unmap_gfn_range(struct vm_area_struct *vma,
>>                     int nr, struct page **pages)
>>   {
>>       int i;
>> +    int nr_page = round_up(nr, XEN_PFN_PER_PAGE);
>> -    for (i = 0; i < nr; i++) {
>> -        struct xen_remove_from_physmap xrp;
>> -        unsigned long pfn;
>> +    for (i = 0; i < nr_page; i++) {
>> +        /* unmap_gfn guarantees ret == 0 */
>> +        BUG_ON(xen_apply_to_page(pages[i], unmap_gfn, &nr));
>
>
> TBH, I am not sure how useful xen_apply_to_page() routine is. In this
> patch especially, but also in others.

It avoids an open loop in each place where it's needed (here, 
balloon...) which means another indentation layer. You can give a look 
it's quite ugly.

Furthermore, the helper will avoid possible done by developers who are 
working on PV drivers on x86. If you see code where the MFN translation 
is done directly via virt_to_mfn or page_to_mfn... it will likely means 
that the code will be broking when 64KB page granularity will be in used.
Though, there will still be some place where it's valid to use 
virt_to_mfn and page_to_mfn.

Regards,

-- 
Julien Grall
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