linux-kernel - Re: kvm vmload/vmsave vs tss.ist

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Message-ID: <4953AAE5.4000708@redhat.com>
Date:	Thu, 25 Dec 2008 17:46:45 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Benjamin Serebrin <benjamin.serebrin@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	Alexander Graf <agraf@...e.de>
Subject: Re: kvm vmload/vmsave vs tss.ist

Ingo Molnar wrote:
> i think we should actually do #1 unconditionally.
>
> ISTs are bad for the native kernel too. They have various nasty 
> complications in the stack walker (and hence they _reduce_ reliability in 
> practice), and they are non-preemptible as well. Plus we have the 
> maximum-stack-footprint ftrace plugin now, which can remove any perception 
> about how bad the worst-case stack footprint is in practice.
>
> If it ever becomes an issue we could also soft-switch to a larger (per 
> CPU) exception stack from the exception handlers themselves. The 
> architectural stack footprint of the various critical exceptions are 
> calculatable and low - so we could switch away and get almost the kind of 
> separation that ISTs give. There's no deep reason to actually make use of 
> hw switched ISTs.
>
> So feel free to send a patch that just standardizes the critical 
> exceptions to use the regular kernel stack. (I havent actually tried this 
> but it should be relatively simple to implement. Roadblocks are possible.)
>   

Certainly.  There is provision for a debug stack that can be larger than 
the normal exception stack.  This is used for vectors 1 and 3.  If we 
wish to preserve this, we need to to manual stack switching.

Currently DEBUG_STKSZ is 8K, the same as the normal stack (compared to 
4K for the other execption stacks).  Do we need to implement stack 
switching for debug vectors?

-- 
error compiling committee.c: too many arguments to function

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