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xen: cache cr0 value to avoid trap'n'emulate for read_cr0

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stts() is implemented in terms of read_cr0/write_cr0 to update the
state of the TS bit.  This happens during context switch, and so
is fairly performance critical.  Rather than falling back to
a trap-and-emulate native read_cr0, implement our own by caching
the last-written value from write_cr0 (the TS bit is the only one
we really care about).

Impact: optimise Xen context switches
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
This commit is contained in:
Jeremy Fitzhardinge 2009-04-24 00:26:50 -07:00
parent b80119bb35
commit a789ed5fb6
1 changed files with 17 additions and 1 deletions
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18
arch/x86/xen/enlighten.c
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@ -658,10 +658,26 @@ static void xen_clts(void)
xen_mc_issue(PARAVIRT_LAZY_CPU);
}
static DEFINE_PER_CPU(unsigned long, xen_cr0_value);
static unsigned long xen_read_cr0(void)
{
unsigned long cr0 = percpu_read(xen_cr0_value);
if (unlikely(cr0 == 0)) {
cr0 = native_read_cr0();
percpu_write(xen_cr0_value, cr0);
}
return cr0;
}
static void xen_write_cr0(unsigned long cr0)
{
struct multicall_space mcs;
percpu_write(xen_cr0_value, cr0);
/* Only pay attention to cr0.TS; everything else is
ignored. */
mcs = xen_mc_entry(0);
@ -847,7 +863,7 @@ static const struct pv_cpu_ops xen_cpu_ops __initdata = {
.clts = xen_clts,
.read_cr0 = native_read_cr0,
.read_cr0 = xen_read_cr0,
.write_cr0 = xen_write_cr0,
.read_cr4 = native_read_cr4,