Re: [PATCH v5 2/6] x86: allow reading MSR_IA32_TSC with XENPF_resource_op

[Chao Peng <chao.p.peng@linux.intel.com>]

On Thu, Jan 22, 2015 at 02:09:37PM +0000, Jan Beulich wrote:
> >>> On 22.01.15 at 15:03, <andrew.cooper3@citrix.com> wrote:
> > On 22/01/15 13:36, Jan Beulich wrote:
> >>>>> On 22.01.15 at 13:53, <chao.p.peng@linux.intel.com> wrote:
> >>> On Thu, Jan 22, 2015 at 11:20:15AM +0000, Jan Beulich wrote:
> >>>>>>> On 21.01.15 at 12:19, <chao.p.peng@linux.intel.com> wrote:
> >>>>> --- a/xen/arch/x86/platform_hypercall.c
> >>>>> +++ b/xen/arch/x86/platform_hypercall.c
> 
> > Having said this, the only useful timestamp will be at the same point as
> > performing the MSR read.  Having a 3rd operation tacked on the end to
> > get a timestamp will be some arbitrary time later, especially if
> > interrupts are enabled.
> 
> Perhaps the latching of NOW() could happen with the MSR read, and
> the latched value then be stored upon encountering the respective
> slot? That would also allow further limiting the interrupts disabled
> period.

Except that MSR_IA32_TSC will looks heterogeneous. But since we already
treat it as a special case, I have no problem here. And if we do so,
even the first patch to add irq_disable ability is not needed. Sounds to
me that the MSR_IA32_TSC read should always imply irq disabled. Codes
would be like this:

        case XEN_RESOURCE_OP_MSR_READ:
            if ( unlikely(entry->idx == MSR_IA32_TSC) )
            { 
                entry->val = get_s_time_fixed(tsc);
                ret = 0;
            }   
            else
            {
                bool_t  read_tsc = 0;
                if ( i < ra->nr_done - 1 ) 
                {   
                    xenpf_resource_entry_t *next = ra->entries + i + 1;
                    if ( unlikely(next->idx == MSR_IA32_TSC) )
                        read_tsc = 1;
                }

                if ( unlikely(read_tsc) )
                    local_irq_save(irqflags);

                ret = rdmsr_safe(entry->idx, entry->val);

                if ( unlikely(read_tsc) )
                {
                    rdtscll(tsc);
                    local_irq_restore(irqflags);
                }
            }
            break;
        case XEN_RESOURCE_OP_MSR_WRITE:

Chao