Re: Xen 4.3 development update

[George Dunlap <george.dunlap@eu.citrix.com>]

On 04/05/13 11:47, Pasi Kärkkäinen wrote:
> On Fri, May 03, 2013 at 05:41:35PM +0100, George Dunlap wrote:
>> On 02/05/13 16:48, Tim Deegan wrote:
>>> At 15:21 +0200 on 29 Apr (1367248894), Peter Maloney wrote:
>>>> On 04/04/2013 07:05 PM, Tim Deegan wrote:
>>>>> Also, if there is still a bad slowdown, caused by the p2m lookups, this
>>>>> might help a little bit:
>>>>>
>>>>> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
>>>>> index 38e87ce..7bd8646 100644
>>>>> --- a/xen/arch/x86/hvm/hvm.c
>>>>> +++ b/xen/arch/x86/hvm/hvm.c
>>>>> @@ -1361,6 +1361,18 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
>>>>>           }
>>>>>       }
>>>>> +
>>>>> +    /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
>>>>> +     * a fast path for LAPIC accesses, skipping the p2m lookup. */
>>>>> +    if ( !nestedhvm_vcpu_in_guestmode(v)
>>>>> +         && gfn == vlapic_base_address(vcpu_vlapic(current)) >> PAGE_SHIFT )
>>>>> +    {
>>>>> +        if ( !handle_mmio() )
>>>>> +            hvm_inject_hw_exception(TRAP_gp_fault, 0);
>>>>> +        rc = 1;
>>>>> +        goto out;
>>>>> +    }
>>>>> +
>>>>>       p2m = p2m_get_hostp2m(v->domain);
>>>>>       mfn = get_gfn_type_access(p2m, gfn, &p2mt, &p2ma,
>>>>>                                 P2M_ALLOC | (access_w ? P2M_UNSHARE : 0), NULL);
>>>> This patch (applied to 4.2.2) has a very large improvement on my box
>>>> (AMD FX-8150) and WinXP 32 bit.
>>> Hmm - I expected it to be only a mild improvement.  How about this one,
>>> which puts in the same shortcut in another place as well?  I don't think
>>> it will be much better than the last one, but it's worth a try.
>> So I dusted off my old perf testing scripts and added in one to
>> measure boot performance.
>>
>> Below are boot times, from after "xl create" returns, until a
>> specific python daemon running in the VM starts responding to
>> requests.  So lower is better.
>>
>> There are a number of places where there can be a few seconds of
>> noise either way, but on the whole the tests seem fairly repeatable.
>>
>> I ran this with w2k3eesp2 and with winxpsp3, using some of the
>> auto-install test images made for the XenServer regression testing.
>> All of them are using a flat file disk backend with
>> qemu-traditional.
>>
>> Results are in order of commits:
>>
>> Xen 4.1:
>>
>> w2k3: 43 34 34 33 34
>> winxp: 110 111 111 110 112
>>
>> Xen 4.2:
>>
>> w2k3: 34 44 45 45 45
>> winxp: 203 221 210 211 200
>>
>> Xen-unstable w/ RTC fix:
>>
>> w2k3: 43 44 44 45 44
>> winxp: 268 275 265 276 265
>>
>> Xen-unstable with rtc fix + this "fast lapic" patch:
>>
>> w2k3: 43 45 44 45 45
>> winxp: 224 232 232 232 232
>>
>>
>> So w2k3 boots fairly quickly anyway; has a 50% slow-down when moving
>> from 4.1 to 4.2, and no discernible change after that.
>>
>> winxp boots fairly slowly; nearly doubles in speed for 4.2, and gets
>> even worse for xen-unstable.  The patch is a measurable improvement,
>> but still nowhere near 4.1, or even 4.2.
>>
>> On the whole however -- I'm not sure that boot time by itself is a
>> blocker.  If the problem really is primarily the "eager TPR" issue
>> for Windows XP, then I'm not terribly motivated either: the Citrix
>> PV drivers patch Windows XP to modify the routine to be lazy (like
>> w2k3); there is hardware available which allows the TPR to be
>> virtualized; and there are plenty of Windows-based OSes available
>> which do not have this problem.
>>
> A couple of questions:
>
> - Does Citrix XenServer Windows PV driver work with vanilla Xen 4.2.x? I remember someone complaining on the list that it doesn't work.. (but I'm not sure about that).

I did a quick test of the XS 6.0.2 drivers on unstable and they didn't 
work.  Didn't do any debugging, however.

> - Does GPLPV do the lazy patching for WinXP on AMD?

I highly doubt it, but you'd have to ask James Harper.

  -George