From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: What happens on an INT80 instruction Date: Tue, 02 Oct 2007 09:25:49 -0500 Message-ID: <470254ED.5030408@codemonkey.ws> References: <47004DDE.1060603@codemonkey.ws> <47024C4D.6060302@codemonkey.ws> <470251F9.7030902@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel To: Avi Kivity Return-path: In-Reply-To: <470251F9.7030902-atKUWr5tajBWk0Htik3J/w@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Avi Kivity wrote: > Anthony Liguori wrote: >> Jun Koi wrote: >> >>> On 10/1/07, Anthony Liguori wrote: >>> >>>> Cameron Macdonell wrote: >>>> >>>>> Hi, >>>>> >>>>> I'm trying to understand guest virtualization at the lower levels. I >>>>> have a somewhat basic question: How does KVM virtualize an int80 >>>>> instruction from a guest? A pointer to an answer is just as good as >>>>> an answer itself. >>>>> >>>>> >>>> The same thing happens as it does on normal hardware. >>>> >>>> The way VT/SVM works (at a high level), is that certain >>>> instructions and >>>> events check a special area called the VMCS/VMCB to determine whether >>>> the event should generate a vmexit which is really just a special type >>>> of trap. >>>> >>>> There are no hooks for interrupts 32-255 so the hardware operates >>>> as it >>>> normally would. If you're interested in getting a trap for int80 >>>> within >>>> KVM, you'll have to trap sidt/lidt and virtualize the IDT. You'll >>>> need >>>> to setup a fake IDT and have the int80 handler do a hypercall. >>>> This is >>>> complicated if the guest is using a fast-syscall mechanism. It may >>>> be a >>>> little challenging finding a piece of guest memory to take over >>>> that has >>>> a valid virtual mapping. >>>> >>> This is a bit vague to me. Why do you need "a piece of guest memory" >>> here? >>> >> >> You don't just need guest memory, you need a valid guest virtual >> address too. The IDTR contains a guest VA. If you want to create >> your own IDT, then it has to be a valid VA in the guest's address space. >> >> > > You can set the guest idt size to zero and trap the double fault > exception. You could, but then you're trapping all exceptions instead of just the int80. Of course, int80 is probably the one you care most about performance wise so it's probably a reasonable approach. Regards, Anthony Liguori ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/