Re: [Xen-devel] [PATCH for-next 7/7] x86: implement Hyper-V clock source

[Wei Liu <wl@xxxxxxx>]

On Wed, Dec 18, 2019 at 01:51:54PM +0100, Jan Beulich wrote:
> On 18.12.2019 13:38, Wei Liu wrote:
> > On Tue, Dec 10, 2019 at 05:59:04PM +0100, Jan Beulich wrote:
> >> On 25.10.2019 11:16, Wei Liu wrote:
> >>> +static inline uint64_t read_hyperv_timer(void)
> >>> +{
> >>> +    uint64_t scale, offset, ret, tsc;
> >>> +    uint32_t seq;
> >>> +    struct ms_hyperv_tsc_page *tsc_page = &hyperv_tsc_page;
> >>> +
> >>> +    do {
> >>> +        seq = tsc_page->tsc_sequence;
> >>> +
> >>> +        /* Seq 0 is special. It means the TSC enlightenment is not
> >>> +         * available at the moment. The reference time can only be
> >>> +         * obtained from the Reference Counter MSR.
> >>> +         */
> >>> +        if ( seq == 0 )
> >>> +        {
> >>> +            rdmsrl(HV_X64_MSR_TIME_REF_COUNT, ret);
> >>> +            return ret;
> >>> +        }
> >>> +
> >>> +        smp_rmb();
> >>> +
> >>> +        tsc = rdtsc_ordered();
> >>
> >> This already includes at least a read fence.
> > 
> > OK. rdtsc() should be enough here.
> 
> Are you sure? My comment was rather towards the dropping of smp_rmb()
> (maybe replacing by a comment).

I do mean to keep smp_rmb() before it. Is that not enough?

Wei.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel