Re: [Xen-devel] [PATCH v2 0/6] add xsaves/xrstors support

["Ruan, Shuai" <shuai.ruan@xxxxxxxxx>]

Hi Andrew：
Thanks for your review.
In V2 , I have test migrate VM between two different servers, one which is 
xsaves capable and one which is not. There is 2 bugs related with my code in 
migrating between two servers( in v1 I just test migrate using localhost) and I 
fix it by add change the code.
I forgot to mention it in v2 0/6.

Thanks

shuai

-----Original Message-----
From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx] 
Sent: Saturday, July 18, 2015 4:15 AM
To: Ruan, Shuai; xen-devel@xxxxxxxxxxxxx
Cc: ian.jackson@xxxxxxxxxxxxx; Ian.Campbell@xxxxxxxxxx; 
stefano.stabellini@xxxxxxxxxxxxx; wei.liu2@xxxxxxxxxx; jbeulich@xxxxxxxx; 
Nakajima, Jun; keir@xxxxxxx; Dong, Eddie; Tian, Kevin
Subject: Re: [PATCH v2 0/6] add xsaves/xrstors support

On 17/07/15 08:26, Shuai Ruan wrote:
> Changes in v2:
> * Address comments from Andrew/chao/Jan, mainly:
> * Add details information of xsaves/xrstors feature.
> * Remove XGETBV1/XSAVEC/XSAVEOPT out of 'else' in patch 3.
> * Change macro name XGETBV to XGETBV1 in patch 4.
>
> This patchset enable xsaves/xrstors feature which will be available on 
> Intel Skylake and later platform. Like xsave/xrstor, xsaves/xrstors 
> feature will save and load processor state from a region of memory 
> called XSAVE area. While unlike xsave/xrstor, xsaves/xrstors:
>
> a) use the compacted format only for the extended region 
>    of the XSAVE area which saves memory for you;
> b) can operate on supervisor state components so the feature
>    is prerequisite to support new supervisor state components;
> c) execute only if CPL=0. 
>
> Detail hardware spec can be found in chapter 13 (section 13.11 13.12) 
> of the Intel SDM [1].
>
> patch1: add xsaves/xrstors support for pv guest
> patch2: add xsaves/xrstors support for xen
> patch3-5: add xsaves/xrstors support for hvm guest
> patch6: swtich on detection of xsaves/xrstors/xgetbv in xen
>
> [1] Intel SDM 
> (http://www.intel.com/content/dam/www/public/us/en/documents/manuals/6
> 4-ia-32-architectures-software-developer-manual-325462.pdf)

Thankyou for this - it is far more useful 0/$N now.

However, looking at the series, you clearly have not tried migrating a VM 
between two different servers, one which is xsavec capable and one which is not.

The reason why Xen does not currently use compressed xsave areas is that it has 
an ABI to maintain which predates compression.  In some cases there is no 
question; the uncompressed format must be used.  In other cases such as 
migration itself, compressed format could be used, but may not assume that the 
far side of the migration has hardware capable of processing the compressed 
format.

~Andrew
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