Re: Why exit on MSR_STAR and friends?

["Nadav Har'El" <nyh@math.technion.ac.il>]

On Thu, Feb 17, 2011, Avi Kivity wrote about "Re: Why exit on MSR_STAR and friends?":
> It isn't needed.  The code doesn't distinguish between the read and 
> write bitmaps, and so far no guest issues rdmsr for these msrs with any 
> frequency (kvm as a guest will write those msrs, but it shouldn't read 
> them on Intel).  Do you see frequent reads on some guest?

I saw reads of all these MSRs (STAR, LSTAR, CSTAR and SYSCALL_MASK) at about
half the number of the writes.

Looking deeper now, I realize why I saw these and you didn't. I happened
to run some old L1 image, with apparently a 1 year old Linux and KVM.
In that version, __vmx_load_host_state() called:
	save_msrs(vmx->guest_msrs, vmx->save_nmsrs);
        load_msrs(vmx->host_msrs, vmx->save_nmsrs);
and save_msrs read all those MSRs. Looking at the current code, indeed
this is no loger done.

So you can say that there are some guests which frequently reads those MSRs -
old versions of KVM :-) But I agree, this is nothing to worry about.

I guess I should be more worried why I got all these PIOs in L2 in the
first place - the MSR reads and writes in L1 were just an odd consequence
of that. It turns out that the "ping -f" workload I was running in L2
insisted to get accurate timings of each packet, and this, I'm not still
sure why, caused a ACPI PM_TIMER PIO for each packet. I guess that normal
workloads won't use the timer on every packet, so that shouldn't matter.

-- 
Nadav Har'El                        |    Thursday, Feb 17 2011, 13 Adar I 5771
nyh@math.technion.ac.il             |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |Knowledge is knowing a tomato is a fruit.
http://nadav.harel.org.il           |Wisdom is not putting it in a fruit salad.