Re: [Xen-devel] [PATCH] VMX: wbinvd when vmentry under UC

["Auld, Will" <will.auld@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, November 25, 2013 8:47 AM
> To: Andrew Cooper; Liu, Jinsong
> Cc: sherry.hurwitz@xxxxxxx; suravee.suthikulpanit@xxxxxxx; Dugger,
> Donald D; Dong, Eddie; Nakajima, Jun; Auld, Will; Zhang, Xiantao; xen-
> devel@xxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx; zhenzhong.duan@xxxxxxxxxx;
> keir@xxxxxxx; tim@xxxxxxx
> Subject: Re: [PATCH] VMX: wbinvd when vmentry under UC
> 
> >>> On 25.11.13 at 17:39, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> wrote:
> > On 25/11/13 16:14, Liu, Jinsong wrote:
> >> +/*
> >> + * wbinvd is a _very_ time consuming operation, so
> >> + * 1. wbinvd ... timer has a good possibility to expire while
> >> + *    irq disabled, it then would be delayed until
> >> + * 2. ... vmentry back to guest (and irq enalbed), timer interrupt
> >> + *    then occurs and drops guest at once;
> >> + * 3. drop to hypervisor ... then vmentry and wbinvd again;
> >> + *
> >> + * This loop will run again and again, until lucky enough wbinvd
> >> + * happens not to expire timer and then loop break, usually it
> would
> >> + * occur 10K~60K times, blocking guest 10s~60s.
> >> + *
> >> + * reprogram timer to avoid dead_like_loop.
> >> + */
> >> +static inline void uc_wbinvd_and_timer_adjust(void) {
> >> +    reprogram_timer(0);
> >> +    wbinvd();
> >> +    reprogram_timer(NOW() + MILLISECS(1));
> >
> > Do you have any number of the time delta across the wbinvd() ?
> >
> > As it currently stands, I do not think it is reasonable to reprogram
> > the timer like this.
> 
> Indeed I was wondering too, but didn't get to look in detail at what
> consequences would arise from doing this.
> 
> Jan

Basically, increase the timer setting so that it is unlikely to fire during 
wbinvd() but still be there as a safeguard. Then reset as you are currently 
doing after wbinvd(). 

Will

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