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RE: [Xen-devel] Improving hvm IO performance by using self IO emulator(YA io-emu?)

  • To: "Tristan Gingold" <tgingold@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Guy Zana" <guy@xxxxxxxxxxxx>
  • Date: Thu, 22 Feb 2007 05:34:22 -0500
  • Delivery-date: Thu, 22 Feb 2007 02:35:18 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcdWQRa8m7Pm8+ClTRuyEl5zxoNxUQAKbD7g
  • Thread-topic: [Xen-devel] Improving hvm IO performance by using self IO emulator(YA io-emu?)

Are you suggesting to write EFI PV Drivers?

This method sounds very promising, but there are some limitations, Windows 
Vista 32bit does not support EFI (so I've read).
It's like PV-on-HVM but it also eliminates the need to install the regular 
PV-on-HVM drivers.

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Tristan Gingold
> Sent: Thursday, February 22, 2007 7:23 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Improving hvm IO performance by using 
> self IO emulator(YA io-emu?)
> Summary: I am proposing  a new method to improve hvm IO 
> emulation: the IO requests are reflected to the domain 
> firmware which emulates the IO using PV drivers.  The pros of 
> this method are minor hypervisor modifications, smooth 
> transition, performance improvement and convergence with PV model
> Discussion:
> The current IO emulator (ioemu process in dom-0) is a well 
> known bottleneck for hvm performance because IO requests 
> travel is long and cross many rings.
> Many ideas to improve the emulation have been proposed.  None 
> of them have been adopted because their approach are too disruptive.
> Based on my recent firmware experience I'd like to propose a 
> new method.
> The principle is rather simple: the hvm domain does all the 
> work.  IO requests are simply reflected to the domain.  When 
> the hypervisor decodes an IO request it sends it to the 
> domain using a SMI(x86)/PMI(ia64)-like interruption.  This 
> reflection saves some registers, put parameters (IO req) into 
> registers and call the firmware at defined address using a 
> defined mode (physical mode should be the best).  The 
> firmware handles the IO request like ioemu does but use PV 
> drivers (net, blk, fb...) to access to external resources.  
> It then resumes the domain execution through an hypercall 
> which restores registers and mode.
> I think there are many pros to this approach:
> * the changes in the hypervisor are rather small: only the 
> code to do the reflection has to be added.  This is a 
> well-known and light mechanism.
> * the transition can be smooth: this new method can co-exist 
> in several way with the current method.  First it can be used 
> only when enabled.  Then once the reflection code is added in 
> the hypervisor the firmware can just send the IO request to 
> ioemu like the hypervisor already does.  The in domain IO 
> emulation can be added driver per driver (eg: IDE disk first, 
> then network, then fb).
> This smooth transition is a major advantage to early evaluate 
> this new method.
> * Because all the emulation work is done in the domain the 
> work in accounted to this domain and not to another domain 
> (dom0 today).  This is good for management and for security.
> * From the hypervisor point of view such an hvm domain looks 
> like a PV domain:
> only the creation differs.  This IO emulation method unifies 
> the domain.  This will simplify save & restore and Xen in general.
> * Performance should be improved compared to the current io 
> emulation method:
> the IO request travel is shorter.  If we want to work on 
> performance we could later handle directly some IO requests 
> in the hypervisor (I think of ports or iomem which don't have 
> side-effect).
> I don't see a lot of cons, the major one is 'porting' ioemu 
> code to firmware code.  This is the challenge.  But qemu 
> seems to be well structured.
> Most of the files might be ported without changes, the core 
> has of course to be rewritten.  The PV drivers should also be ported.
> SMP can be first handled with a global lock and later 
> concurrent accesses may be allowed.  This may improve 
> performance compared to ioemu which is almost single threaded.
> I don't know yet how to use the PV-on-HVM drivers.  There is 
> currently only one page to communicate with xenstore.  We can 
> try to share this page between the firmware and the PV-on-HVM 
> drivers or we may create a second page.
> I have thought of this new IO emulation method during my work 
> on EFI gfw for ia64.  Recently I have looked more deeply into 
> the sources.  I can't see any stopper yet.  Unless someone 
> has a strong point against this method I hope I will be able 
> to work on it shortly (ia64 first - sorry!)
> Comments are *very* welcome.
> Tristan.
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