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Re: [Xen-devel] Question about HLT VMExit reason

I think thatâs an answer to why hlt is a vmexit as opposed to when it is likely to occurâ


This simple answer, of course, is the OS idle process will most likely the issuer of a hlt instruction and this is why you are seeing them so frequently. Give your guest some compute-bound task and youâll  mostly likely see a big fall-off in the number of hlts.




From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of Andrew Cooper
Sent: 12 February 2015 10:07
To: zhangtrump; xen-devel@xxxxxxxxxxxxx
Subject: Re: [Xen-devel] Question about HLT VMExit reason


On 12/02/15 09:24, zhangtrump wrote:

Hi, all:


     I was testing networking performance from DomU to DomU running on different hosts. I used xentrace and xenalyze tools to get the trace info of receive side, and found that there was a time when vcpus (which handle the receive work) were in "Blocked" state.

     From the information of "VMExit" reason gotten by xenalyze tool, the "HLT" is the main exit reason. I am wondering when DomU will be exited because of "HLT"? 



When one vcpu halts, Xen will schedule another vcpu in its place.  There is nothing to be gained at all by letting a pcpu stay halted in guest.


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