RE: [Xen-devel] hvm iret [also xen interrupt handling deltails]

["Cui, Dexuan" <dexuan.cui@xxxxxxxxx>]

> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Ashish Bijlani
> Sent: 2007年10月21日 21:59
> To: Xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] hvm iret [also xen interrupt handling deltails]

> i'd like to know is what happens when a guest(hvm) OS krnl executes iret? 
> does it trap to xen?
Usually no trap to Xen. The execution of guest OS is just restored to where it 
was interrupted --  however, if there is pending vINTR/vNMI, and no vINTR/vNMI 
blocking any longer and Xen set some VIRTUAL_INTR/NMI_PENDING, certain kinds of 
VMexits can occur before guest continues to run, allowing Xen to inject the 
pending events in time.

> would it _always_ trap to xen in  paravirtualized environment ( i.e. no 
> hardware assisted virtualization)? 
> also, i understand that when a physical interrupt comes, xen sets bits in 
> some event bitmask to inject events (or virtual interrupts) into
> guest VMs, but when does xen send EOI to the IOAPIC? does it send after all 
> the guest VMs have executed their ISRs or does it send
> immediately upon receiving a physical interrupt? thanks again. 
Sorry, I'm not very sure about everything for the case of para-guest.
For HVM guest in Xen (here we don't consider VT-d), no physical devices are 
owned by HVM guest at all. The devices HVM guest sees are all emulated by the 
Qemu process in Dom0 -- from this perspective,  we don't care when/how Xen 
sends physical EOI to physical IOAPIC.

-- Dexuan

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