Re: [Xen-devel] Hypercalls in Intel-VT

["Ashish Bijlani" <ashish.bijlani@xxxxxxxxx>]

could there be a way to make a transition into xen without a vmexit upon executing an iret inside a guest os krnl?

On 10/25/07, Cui, Dexuan < dexuan.cui@xxxxxxxxx> wrote:

> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Ashish Bijlani
> Sent: 2007年10月26日 7:44
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Hypercalls in Intel-VT

> if I push xen's some addr (cs & eip) on the target guest os krnl stack when an interrupt occurs, would the guest os krnl return to xen
> immediately upon executing iret (as iret pops cs, eip from the krnl stack)? or would it make a vmexit coz of some illegal addr access

I assume you're only talking about Intel VMX guest. The answer is NO.
For instance, a VMX guest is running in "non-root mode"; it can only use its (shadowed) page table; it can't use IRET to return to Xen that's running on "root mode".
If you did that, what would happen is, i.e., a 32-bit protection-mode guest uses its own GDT -- the guest may find the GDT entry indexed by Xen's CS is invalid, so a #GP is generated in guest directly, and guest handles this (No VMexit).

-- Dexuan

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