Re: [PATCH for-4.5] tools/libxc: Avoid cacheflush toolstack hypercalls on x86

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 24/09/14 15:22, Ian Campbell wrote:
> On Wed, 2014-09-24 at 14:47 +0100, Andrew Cooper wrote:
>> XEN_DOMCTL_cacheflush hypercalls are (and will always be) -ENOSYS on x86, but
>> xc_domain_cacheflush() is called often during domain build and migrate for
>> correct behaviour on ARM.
>>
>> Stub xc_domain_cacheflush() out on x86 to remove its pressure on the global
>> domctl lock, and the hypercall overhead (which applies further pressure to the
>> already heavily-contended TLB flush lock).
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> CC: Keir Fraser <keir@xen.org>
>> CC: Jan Beulich <JBeulich@suse.com>
>> CC: Tim Deegan <tim@xen.org>
>> CC: Ian Campbell <Ian.Campbell@citrix.com>
>> CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
>> CC: Wei Liu <wei.liu2@citrix.com>
> Acked-by: Ian Campbell <ian.campbell@citrix.com>
>
> Although IMHO only the first paragraph of the comment is useful, the
> rest belongs in the commit log (where it already mostly is already).

I am happy for that to be fixed while committing, or I could resubmit?

~Andrew

>
>> ---
>>  tools/libxc/xc_domain.c |   16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>>
>> diff --git a/tools/libxc/xc_domain.c b/tools/libxc/xc_domain.c
>> index 1a6f90a..a68127f 100644
>> --- a/tools/libxc/xc_domain.c
>> +++ b/tools/libxc/xc_domain.c
>> @@ -51,12 +51,28 @@ int xc_domain_create(xc_interface *xch,
>>  int xc_domain_cacheflush(xc_interface *xch, uint32_t domid,
>>                           xen_pfn_t start_pfn, xen_pfn_t nr_pfns)
>>  {
>> +#if defined (__i386__) || defined (__x86_64__)
>> +    /*
>> +     * The x86 architecture provides cache coherency guarantees which prevent
>> +     * the need for this hypercall.
>> +     *
>> +     * This call however is made frequently during domain build and migrate
>> +     * for the benefit of ARM, where cache flushes are needed.
>> +     *
>> +     * Stub it out here on x86 as domctls take the global domctl lock in Xen
>> +     * which is detrimental to concurrent toolstack operations on other
>> +     * domains.
>> +     */
>> +    errno = ENOSYS;
>> +    return -1;
>> +#else
>>      DECLARE_DOMCTL;
>>      domctl.cmd = XEN_DOMCTL_cacheflush;
>>      domctl.domain = (domid_t)domid;
>>      domctl.u.cacheflush.start_pfn = start_pfn;
>>      domctl.u.cacheflush.nr_pfns = nr_pfns;
>>      return do_domctl(xch, &domctl);
>> +#endif
>>  }
>>  
>>  int xc_domain_pause(xc_interface *xch,
>