Re: [patch 1/2] KVM: x86: handle double and triple faults for every exception

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Nov 11, 2009 at 09:07:08PM +0100, Jan Kiszka wrote:
> Marcelo Tosatti wrote:
> > From: Joerg Roedel <joerg.roedel@amd.com>
> > 
> > The current KVM x86 exception code handles double and triple faults only for
> > page fault exceptions. This patch extends this detection for every exception
> > that gets queued for the guest.
> > 
> > Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
> > CC: Jan Kiszka <jan.kiszka@web.de>
> 
> For a moment I felt like I was time traveling - back in '08. :)
> 
> Reading the archive I noticed that someone posted a fix-up for this patch:
> 
> http://permalink.gmane.org/gmane.comp.emulators.kvm.devel/16931
> 
> Why don't we need this anymore?

I suppose qemu-kvm's call to set_sregs (via system_reset) will end up
clearing pending exception?

> Moreover, are we sure to not regress /wrt to the cases that shall be
> handled serially? So far they should have triggered the WARN_ON, right?

Right. 

How can it regress though, given that serially handled exceptions are
not supported at the moment (you get a WARN_ON and lose the previously
queued anyway).

> But maybe that went through unnoticed, and the guest was simply happy to
> not have triggered a triple fault...

Can't parse. What went through unnoticed, and when/which guest was happy 
to not trigger a triple fault?

BTW, from my understanding of the documentation, the triple fault       
should happen only on an exception during execution of the double fault 
handler, which is not what the pagefault injection code does (or Joerg's
patch and the TSS exceptions in the task switch code).