From: Andrew Cooper Date: Fri, 30 Apr 2021 19:17:55 +0000 (+0100) Subject: x86/xstate: Elide redundant writes in set_xcr0() X-Git-Tag: archive/raspbian/4.16.0+51-g0941d6cb-1+rpi1~2^2~42^2~559 X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=9478ee4a1501c8899307eaa24585f8c6f8d23fa9;p=xen.git x86/xstate: Elide redundant writes in set_xcr0() XSETBV is an expensive instruction as, amongst other things, it involves reconfiguring the instruction decode at the frontend of the pipeline. We have several paths which reconfigure %xcr0 in quick succession (the context switch path has 5, including the fpu save/restore helpers), and only a single caller takes any care to try to skip redundant writes. Update set_xcr0() to perform amortisation automatically, and simplify the __context_switch() path as a consequence. Signed-off-by: Andrew Cooper Reviewed-by: Jan Beulich --- diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c index 4dc27f798e..50a27197b5 100644 --- a/xen/arch/x86/domain.c +++ b/xen/arch/x86/domain.c @@ -1977,9 +1977,7 @@ static void __context_switch(void) memcpy(stack_regs, &n->arch.user_regs, CTXT_SWITCH_STACK_BYTES); if ( cpu_has_xsave ) { - u64 xcr0 = n->arch.xcr0 ?: XSTATE_FP_SSE; - - if ( xcr0 != get_xcr0() && !set_xcr0(xcr0) ) + if ( !set_xcr0(n->arch.xcr0 ?: XSTATE_FP_SSE) ) BUG(); if ( cpu_has_xsaves && is_hvm_vcpu(n) ) diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c index d49413b745..65c720a26a 100644 --- a/xen/arch/x86/xstate.c +++ b/xen/arch/x86/xstate.c @@ -55,11 +55,18 @@ static inline bool xsetbv(u32 index, u64 xfeatures) return lo != 0; } -bool set_xcr0(u64 xfeatures) +bool set_xcr0(u64 val) { - if ( !xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures) ) - return false; - this_cpu(xcr0) = xfeatures; + uint64_t *this_xcr0 = &this_cpu(xcr0); + + if ( *this_xcr0 != val ) + { + if ( !xsetbv(XCR_XFEATURE_ENABLED_MASK, val) ) + return false; + + *this_xcr0 = val; + } + return true; }