From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jan Kiszka <jan.kiszka@siemens.com>
Subject: Re: SVM: vmload/vmsave-free VM exits?
Date: Mon, 13 Apr 2015 20:14:38 +0200
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Cc: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>,
        kvm <kvm@vger.kernel.org>, Jailhouse <jailhouse-dev@googlegroups.com>
To: Avi Kivity <avi.kivity@gmail.com>, Joel Schopp <joel.schopp@amd.com>
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On 2015-04-13 20:07, Avi Kivity wrote:
> On 04/13/2015 08:57 PM, Jan Kiszka wrote:
>> On 2015-04-13 19:48, Avi Kivity wrote:
>>> I think that Xen does (or did) something along the lines of disabling
>>> IST usage (by playing with the descriptors in the IDT) and then
>>> re-enabling them when exiting to userspace.
>> So we would reuse that active stack for the current IST users until
>> then.
> 
> Yes.
> 
>> But I bet there are subtle details that prevent a simple switch at
>> IDT level. Hmm, no low-hanging fruit it seems...
> 
> 
> For sure. It's not insurmountable, but fairly hard.
> 
>>>
>>>> [17] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/26712/
>> That thread proposed the complete IST removal. But, given that we still
>> have it 7 years later,
> 
> Well, it's not as if a crack team of kernel hackers was laboring night
> and day to remove it, but...
> 
>>   I suppose that was not very welcome in general.
> 
> Simply removing it is impossible, or an NMI happening immediately after
> SYSCALL will hit user-provided %rsp.
> 
>> Thanks,
>> Jan
>>
>> PS: For the Jailhouse readers: we don't use IST.
>>
> 
> You don't have userspace, yes?  Only guests?

Exactly. The day someone adds userspace, I guess I'll have to create a
new hypervisor.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux

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