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Re: [Xen-devel] [PATCH 5/5] xen: clean up VPF flags macros
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Mon, 28 Sep 2015 07:23:13 +0200
- Cc: tim@xxxxxxx, wei.liu2@xxxxxxxxxx, Ian.Campbell@xxxxxxxxxx, stefano.stabellini@xxxxxxxxxxxxx, andrew.cooper3@xxxxxxxxxx, ian.jackson@xxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxx, david.vrabel@xxxxxxxxxx, keir@xxxxxxx
- Delivery-date: Mon, 28 Sep 2015 05:23:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On 09/25/2015 05:42 PM, Jan Beulich wrote:
On 25.09.15 at 13:54, <JGross@xxxxxxxx> wrote:
Per-VCPU pause flags in sched.h are defined as bit positions and as
values derived from the bit defines. There is only one user of a value
which can be easily converted to use a bit number as well.
I'm not convinced:
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -172,7 +172,7 @@ void getdomaininfo(struct domain *d, struct
xen_domctl_getdomaininfo *info)
info->max_vcpu_id = v->vcpu_id;
if ( !test_bit(_VPF_down, &v->pause_flags) )
{
- if ( !(v->pause_flags & VPF_blocked) )
+ if ( !test_bit(_VPF_blocked, &v->pause_flags) )
test_bit() is quite a bit more complex an operation than a simple &,
and with (on x86) even constant_test_bit() involving a cast to
a pointer to volatile I'm afraid we can't even hope that compilers
would produce identical code for both in cases like this one (as that
casts limits freedom of the compiler). IOW I'd rather see other
test_bit(_VPF_...) uses converted the inverse way (which as a nice
but minor side effect would yield slightly smaller source code).
What about introducing __test_bit() being a variant which can be
reordered by omitting the volatile modifier? I think this would have
the same effect. And we could still get rid of many double definitions
of the same bit. Even if the mask definition of a bit is not error prone
by relying on the definition of the bit position, it makes it harder to
find all users of this bit.
Juergen
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