Re: [PATCH 2/2] KVM: VMX: Initialize vm86 TSS only once.

[Avi Kivity <avi@redhat.com>]

On 02/27/2011 05:52 PM, Gleb Natapov wrote:
> >
> >  According to my reading of the code, if KVM_SET_TSS_ADDR is not
> >  invoked, the guest would fail both before and after the patch, yes?
> >
> Hmmm. Actually no. Before the patch guest that doesn't use KVM_SET_TSS_ADDR
> will use the top of slot zero. Should I fix that (how?), or should we
> drop support for those old guests?

I don't think we have a problem with older qemus, but perhaps we do with 
non-qemu users.  The API clearly requires the ioctl to be called, but I 
don't think we can blame anyone for forgetting to do so, especially if 
it worked silently.

> The problem with using top of slot
> zero is that this memory is available for guest use and we do not even
> put it into e820 map as far as I see. Also there are patches floating
> around that re-arrange memslots or even put them in a tree. They will
> break old guests too.

Well, slot 0 still exists even if it is moved somewhere else.

Something we can do is put the tss slot just below the highest slot that 
is still below 4G, and hope there is no mmio there.  Once the user 
issues KVM_SET_TSS_ADDR, use that.  We'll have to keep juggling that 
slot as the user creates more slots, icky.

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