linux-kernel - Re: [RFC PATCH 0/3] generic hypercall support

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Date:	Fri, 8 May 2009 12:33:20 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Avi Kivity <avi@...hat.com>, Chris Wright <chrisw@...s-sol.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [RFC PATCH 0/3] generic hypercall support

On Fri, May 08, 2009 at 08:43:40AM -0400, Gregory Haskins wrote:
> The problem is the exit time in of itself isnt all that interesting to
> me.  What I am interested in measuring is how long it takes KVM to
> process the request and realize that I want to execute function "X". 
> Ultimately that is what matters in terms of execution latency and is
> thus the more interesting data.  I think the exit time is possibly an
> interesting 5th data point, but its more of a side-bar IMO.   In any
> case, I suspect that both exits will be approximately the same at the
> VT/SVM level.
> 
> OTOH: If there is a patch out there to improve KVMs code (say
> specifically the PIO handling logic), that is fair-game here and we
> should benchmark it.  For instance, if you have ideas on ways to improve
> the find_pio_dev performance, etc....   

<guess mode on>

One easy thing to try is to cache the last successful lookup on a
pointer, to improve patterns where there's "device locality" (like
nullio test).

<guess mode off>

> One item may be to replace the kvm->lock on the bus scan with an RCU
> or something.... (though PIOs are very frequent and the constant
> re-entry to an an RCU read-side CS may effectively cause a perpetual
> grace-period and may be too prohibitive). CC'ing pmck.

Yes, locking improvements are needed there badly (think for eg the cache
bouncing of kvm->lock _and_ bouncing of kvm->slots_lock on 4-way SMP
guests).

> FWIW: the PIOoHCs were about 140ns slower than pure HC, so some of that
> 140 can possibly be recouped.  I currently suspect the lock acquisition
> in the iobus-scan is the bulk of that time, but that is admittedly a
> guess.  The remaining 200-250ns is elsewhere in the PIO decode.

vmcs_read is significantly expensive
(http://www.mail-archive.com/kvm@vger.kernel.org/msg00840.html,
likely that my measurements were foobar, Avi mentioned 50 cycles for
vmcs_write).

See for eg how vmx.c reads VM_EXIT_INTR_INFO twice on every exit.

Also this one looks pretty bad for a 32-bit PAE guest (and you can 
get away with the unconditional GUEST_CR3 read too).

        /* Access CR3 don't cause VMExit in paging mode, so we need
         * to sync with guest real CR3. */
        if (enable_ept && is_paging(vcpu)) {
                vcpu->arch.cr3 = vmcs_readl(GUEST_CR3);
                ept_load_pdptrs(vcpu);
        }

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