linux-kernel - Re: [Xen-devel] [PATCH V2 5/5] Xen: switch to linear virtual mapped sparse p2m list

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Message-ID: <545CDAA8.4050803@suse.com>
Date:	Fri, 07 Nov 2014 15:43:52 +0100
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com, x86@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Subject: Re: [Xen-devel] [PATCH V2 5/5] Xen: switch to linear virtual mapped
 sparse p2m list

On 11/07/2014 03:40 PM, David Vrabel wrote:
> On 07/11/14 14:11, Juergen Gross wrote:
>> On 11/07/2014 02:54 PM, David Vrabel wrote:
>>> On 06/11/14 05:47, Juergen Gross wrote:
>>> [...]
>>>> @@ -526,23 +411,83 @@ unsigned long get_phys_to_machine(unsigned long
>>>> pfn)
>>>>            return IDENTITY_FRAME(pfn);
>>>>        }
>>>>
>>>> -    topidx = p2m_top_index(pfn);
>>>> -    mididx = p2m_mid_index(pfn);
>>>> -    idx = p2m_index(pfn);
>>>> +    ptep = lookup_address((unsigned long)(xen_p2m_addr + pfn), &level);
>>>> +    BUG_ON(!ptep || level != PG_LEVEL_4K);
>>>>
>>>>        /*
>>>>         * The INVALID_P2M_ENTRY is filled in both p2m_*identity
>>>>         * and in p2m_*missing, so returning the INVALID_P2M_ENTRY
>>>>         * would be wrong.
>>>>         */
>>>> -    if (p2m_top[topidx][mididx] == p2m_identity)
>>>> +    if (pte_pfn(*ptep) == PFN_DOWN(__pa(p2m_identity)))
>>>>            return IDENTITY_FRAME(pfn);
>>>>
>>>> -    return p2m_top[topidx][mididx][idx];
>>>> +    return xen_p2m_addr[pfn];
>>>
>>> You should test xen_p2m_addr[pfn] == INVALID_P2M_ENTRY before checking
>>> if it's an identity entry.  This should skip the more expensive
>>> lookup_address() in the common case.
>>
>> I do. The check is in __pfn_to_mfn(). get_phys_to_machine() is called in
>> this case only.
>
> So you do.  I missed that.
>
>>>>    bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
>>>
>>> I think you should map p2m_missing and p2m_identity as read-only and do
>>> the new page allocation on a write fault.
>>>
>>> set_phys_to_machine() is used every grant map and unmap and in the
>>> common case (already allocated array page) it must be a fast and simple:
>>>
>>>       xen_p2m_addr[pfn] = mfn;
>>
>> Nice idea. I'll try it.
>
> You probably want to try this with a custom exception fixup (or abuse
> put_user()).

Yeah, that was my idea, too.

Juergen

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