From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH] x86: reduce redundancy in tsc_[gs]et_info() Date: Wed, 30 Apr 2014 15:48:47 +0100 Message-ID: <53610D4F.3010005@citrix.com> References: <53611C5D020000780000DCAC@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============2865538698014240333==" Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1WfVoa-00054f-MT for xen-devel@lists.xenproject.org; Wed, 30 Apr 2014 14:48:53 +0000 In-Reply-To: <53611C5D020000780000DCAC@mail.emea.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: xen-devel , Keir Fraser List-Id: xen-devel@lists.xenproject.org --===============2865538698014240333== Content-Type: multipart/alternative; boundary="------------000001040402050808060101" --------------000001040402050808060101 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit On 30/04/14 14:53, Jan Beulich wrote: > - some of the case statements are effectively or mostly special cases > of others, so there's no good reason not to share the code > - in the "get" function, a variable can be made case-wide instead of > having multiple instance of it (and those even with a pointless > initializer) > - minor formatting adjustments > > Signed-off-by: Jan Beulich > > --- a/xen/arch/x86/time.c > +++ b/xen/arch/x86/time.c > @@ -1786,26 +1786,22 @@ void tsc_get_info(struct domain *d, uint > > switch ( *tsc_mode ) > { > + uint64_t tsc; > + > case TSC_MODE_NEVER_EMULATE: > - *elapsed_nsec = *gtsc_khz = 0; > + *elapsed_nsec = *gtsc_khz = 0; > break; > - case TSC_MODE_ALWAYS_EMULATE: > - *elapsed_nsec = get_s_time() - d->arch.vtsc_offset; > - *gtsc_khz = d->arch.tsc_khz; > - break; > case TSC_MODE_DEFAULT: > if ( d->arch.vtsc ) > { > + case TSC_MODE_ALWAYS_EMULATE: > *elapsed_nsec = get_s_time() - d->arch.vtsc_offset; > *gtsc_khz = d->arch.tsc_khz; double space after = > + break; > } > - else > - { > - uint64_t tsc = 0; > - rdtscll(tsc); > - *elapsed_nsec = scale_delta(tsc,&d->arch.vtsc_to_ns); > - *gtsc_khz = cpu_khz; > - } > + rdtscll(tsc); > + *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns); > + *gtsc_khz = cpu_khz; > break; > case TSC_MODE_PVRDTSCP: > if ( d->arch.vtsc ) > @@ -1815,10 +1811,9 @@ void tsc_get_info(struct domain *d, uint > } > else > { > - uint64_t tsc = 0; > rdtscll(tsc); > - *elapsed_nsec = (scale_delta(tsc,&d->arch.vtsc_to_ns) - > - d->arch.vtsc_offset); > + *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns) - > + d->arch.vtsc_offset; > *gtsc_khz = 0; /* ignored by tsc_set_info */ > } > break; There is a coverity complaint that following this switch statement, there is a read of *elapsed_nsec which might be uninitialised if tsc_mode missed on of the case statements. While I believe this is currently impossible, it might we wise to have a "default: BUG();" in case half a new tsc_mode appears. > @@ -1875,28 +1870,24 @@ void tsc_set_info(struct domain *d, > > switch ( d->arch.tsc_mode = tsc_mode ) > { > - case TSC_MODE_NEVER_EMULATE: > - d->arch.vtsc = 0; > - break; > - case TSC_MODE_ALWAYS_EMULATE: > - d->arch.vtsc = 1; > - d->arch.vtsc_offset = get_s_time() - elapsed_nsec; > - d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz; > - set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 ); > - d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns); > - break; > case TSC_MODE_DEFAULT: > - d->arch.vtsc = 1; > + case TSC_MODE_ALWAYS_EMULATE: > d->arch.vtsc_offset = get_s_time() - elapsed_nsec; > - d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz; > - set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 ); > - /* use native TSC if initial host has safe TSC, has not migrated > - * yet and tsc_khz == cpu_khz */ > - if ( host_tsc_is_safe() && incarnation == 0 && > - d->arch.tsc_khz == cpu_khz ) This wont apply cleanly on top of staging. Boris made some changed in this area with his HVM guest TSC series. ~Andrew > + d->arch.tsc_khz = gtsc_khz ?: cpu_khz; > + set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000); > + /* > + * In default mode use native TSC if the host has safe TSC, > + * the VM has not migrated yet, and tsc_khz == cpu_khz. > + */ > + if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() && > + incarnation == 0 && d->arch.tsc_khz == cpu_khz ) > + { > + case TSC_MODE_NEVER_EMULATE: > d->arch.vtsc = 0; > - else > - d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns); > + break; > + } > + d->arch.vtsc = 1; > + d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns); > break; > case TSC_MODE_PVRDTSCP: > d->arch.vtsc = boot_cpu_has(X86_FEATURE_RDTSCP) && > > > > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel --------------000001040402050808060101 Content-Type: text/html; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit
On 30/04/14 14:53, Jan Beulich wrote:
- some of the case statements are effectively or mostly special cases
  of others, so there's no good reason not to share the code
- in the "get" function, a variable can be made case-wide instead of
  having multiple instance of it (and those even with a pointless
  initializer)
- minor formatting adjustments

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1786,26 +1786,22 @@ void tsc_get_info(struct domain *d, uint
 
     switch ( *tsc_mode )
     {
+        uint64_t tsc;
+
     case TSC_MODE_NEVER_EMULATE:
-        *elapsed_nsec =  *gtsc_khz = 0;
+        *elapsed_nsec = *gtsc_khz = 0;
         break;
-    case TSC_MODE_ALWAYS_EMULATE:
-        *elapsed_nsec = get_s_time() - d->arch.vtsc_offset;
-        *gtsc_khz =  d->arch.tsc_khz;
-         break;
     case TSC_MODE_DEFAULT:
         if ( d->arch.vtsc )
         {
+    case TSC_MODE_ALWAYS_EMULATE:
             *elapsed_nsec = get_s_time() - d->arch.vtsc_offset;
             *gtsc_khz =  d->arch.tsc_khz;

double space after =

+            break;
         }
-        else
-        {
-            uint64_t tsc = 0;
-            rdtscll(tsc);
-            *elapsed_nsec = scale_delta(tsc,&d->arch.vtsc_to_ns);
-            *gtsc_khz =  cpu_khz;
-        }
+        rdtscll(tsc);
+        *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns);
+        *gtsc_khz =  cpu_khz;
         break;
     case TSC_MODE_PVRDTSCP:
         if ( d->arch.vtsc )
@@ -1815,10 +1811,9 @@ void tsc_get_info(struct domain *d, uint
         }
         else
         {
-            uint64_t tsc = 0;
             rdtscll(tsc);
-            *elapsed_nsec = (scale_delta(tsc,&d->arch.vtsc_to_ns) -
-                             d->arch.vtsc_offset);
+            *elapsed_nsec = scale_delta(tsc, &d->arch.vtsc_to_ns) -
+                            d->arch.vtsc_offset;
             *gtsc_khz = 0; /* ignored by tsc_set_info */
         }
         break;

There is a coverity complaint that following this switch statement, there is a read of *elapsed_nsec which might be uninitialised if tsc_mode missed on of the case statements.  While I believe this is currently impossible, it might we wise to have a "default: BUG();" in case half a new tsc_mode appears.

@@ -1875,28 +1870,24 @@ void tsc_set_info(struct domain *d,
 
     switch ( d->arch.tsc_mode = tsc_mode )
     {
-    case TSC_MODE_NEVER_EMULATE:
-        d->arch.vtsc = 0;
-        break;
-    case TSC_MODE_ALWAYS_EMULATE:
-        d->arch.vtsc = 1;
-        d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
-        d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
-        set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
-        d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);
-        break;
     case TSC_MODE_DEFAULT:
-        d->arch.vtsc = 1;
+    case TSC_MODE_ALWAYS_EMULATE:
         d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
-        d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
-        set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
-        /* use native TSC if initial host has safe TSC, has not migrated
-         * yet and tsc_khz == cpu_khz */
-        if ( host_tsc_is_safe() && incarnation == 0 &&
-                d->arch.tsc_khz == cpu_khz )

This wont apply cleanly on top of staging.  Boris made some changed in this area with his HVM guest TSC series.

~Andrew

+        d->arch.tsc_khz = gtsc_khz ?: cpu_khz;
+        set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000);
+        /*
+         * In default mode use native TSC if the host has safe TSC,
+         * the VM has not migrated yet, and tsc_khz == cpu_khz.
+         */
+        if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() &&
+             incarnation == 0 && d->arch.tsc_khz == cpu_khz )
+        {
+    case TSC_MODE_NEVER_EMULATE:
             d->arch.vtsc = 0;
-        else 
-            d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);
+            break;
+        }
+        d->arch.vtsc = 1;
+        d->arch.ns_to_vtsc = scale_reciprocal(d->arch.vtsc_to_ns);
         break;
     case TSC_MODE_PVRDTSCP:
         d->arch.vtsc =  boot_cpu_has(X86_FEATURE_RDTSCP) &&






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