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x86/entry: Optimize local_db_save() for virt

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Because DRn access is 'difficult' with virt; but the DR7 read is cheaper
than a cacheline miss on native, add a virt specific fast path to
local_db_save(), such that when breakpoints are not in use to avoid
touching DRn entirely.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200529213321.187833200@infradead.org
This commit is contained in:
Peter Zijlstra
2020-05-29 23:27:36 +02:00
committed by Thomas Gleixner
parent cd840e424f
commit 84b6a34915
3 changed files with 27 additions and 6 deletions
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5
arch/x86/include/asm/debugreg.h
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@@ -85,7 +85,7 @@ static inline void hw_breakpoint_disable(void)
set_debugreg(0UL, 3);
}
static inline int hw_breakpoint_active(void)
static inline bool hw_breakpoint_active(void)
{
return __this_cpu_read(cpu_dr7) & DR_GLOBAL_ENABLE_MASK;
}
@@ -117,6 +117,9 @@ static __always_inline unsigned long local_db_save(void)
{
unsigned long dr7;
if (static_cpu_has(X86_FEATURE_HYPERVISOR) && !hw_breakpoint_active())
return 0;
get_debugreg(dr7, 7);
dr7 &= ~0x400; /* architecturally set bit */
if (dr7)