From 9df9a4ee5129cdb1b185abff94f9c2b7b0e820f9 Mon Sep 17 00:00:00 2001 From: Andrew Cooper Date: Wed, 9 May 2018 13:59:56 +0100 Subject: [PATCH] x86/spec_ctrl: Explicitly set Xen's default MSR_SPEC_CTRL value With the impending ability to disable MSR_SPEC_CTRL handling on a per-guest-type basis, the first exit-from-guest may not have the side effect of loading Xen's choice of value. Explicitly set Xen's default during the BSP and AP boot paths. For the BSP however, delay setting a non-zero MSR_SPEC_CTRL default until after dom0 has been constructed when safe to do so. Oracle report that this speeds up boots of some hardware by 50s. "when safe to do so" is based on whether we are virtualised. A native boot won't have any other code running in a position to mount an attack. Reported-by: Zhenzhong Duan Signed-off-by: Andrew Cooper Reviewed-by: Wei Liu Reviewed-by: Jan Beulich Release-acked-by: Juergen Gross (cherry picked from commit cb8c12020307b39a89273d7699e89000451987ab) --- xen/arch/x86/setup.c | 7 +++++++ xen/arch/x86/smpboot.c | 8 ++++++++ xen/arch/x86/spec_ctrl.c | 32 ++++++++++++++++++++++++++++++++ xen/include/asm-x86/spec_ctrl.h | 2 ++ 4 files changed, 49 insertions(+) diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c index e04a1e4..0ad69d0 100644 --- a/xen/arch/x86/setup.c +++ b/xen/arch/x86/setup.c @@ -1621,6 +1621,13 @@ void __init noreturn __start_xen(unsigned long mbi_p) setup_io_bitmap(dom0); + if ( bsp_delay_spec_ctrl ) + { + get_cpu_info()->spec_ctrl_flags &= ~SCF_use_shadow; + barrier(); + wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl); + } + /* Jump to the 1:1 virtual mappings of cpu0_stack. */ asm volatile ("mov %[stk], %%rsp; jmp %c[fn]" :: [stk] "g" (__va(__pa(get_stack_bottom()))), diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c index 51c24fa..f6abecd 100644 --- a/xen/arch/x86/smpboot.c +++ b/xen/arch/x86/smpboot.c @@ -344,6 +344,14 @@ void start_secondary(void *unused) else microcode_resume_cpu(cpu); + /* + * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard + * any firmware settings. Note: MSR_SPEC_CTRL may only become available + * after loading microcode. + */ + if ( boot_cpu_has(X86_FEATURE_IBRSB) ) + wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl); + smp_callin(); setup_secondary_APIC_clock(); diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c index f31fa6b..c3e940f 100644 --- a/xen/arch/x86/spec_ctrl.c +++ b/xen/arch/x86/spec_ctrl.c @@ -38,6 +38,8 @@ static int8_t __initdata opt_ibrs = -1; static bool_t __initdata opt_rsb_pv = 1; static bool_t __initdata opt_rsb_hvm = 1; bool_t __read_mostly opt_ibpb = 1; + +bool_t __initdata bsp_delay_spec_ctrl; uint8_t __read_mostly default_xen_spec_ctrl; uint8_t __read_mostly default_spec_ctrl_flags; @@ -334,6 +336,36 @@ void __init init_speculation_mitigations(void) __set_bit(X86_FEATURE_SC_MSR_IDLE, boot_cpu_data.x86_capability); print_details(thunk, caps); + + /* + * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard + * any firmware settings. For performance reasons, when safe to do so, we + * delay applying non-zero settings until after dom0 has been constructed. + * + * "when safe to do so" is based on whether we are virtualised. A native + * boot won't have any other code running in a position to mount an + * attack. + */ + if ( boot_cpu_has(X86_FEATURE_IBRSB) ) + { + bsp_delay_spec_ctrl = !cpu_has_hypervisor && default_xen_spec_ctrl; + + /* + * If delaying MSR_SPEC_CTRL setup, use the same mechanism as + * spec_ctrl_enter_idle(), by using a shadow value of zero. + */ + if ( bsp_delay_spec_ctrl ) + { + struct cpu_info *info = get_cpu_info(); + + info->shadow_spec_ctrl = 0; + barrier(); + info->spec_ctrl_flags |= SCF_use_shadow; + barrier(); + } + + wrmsrl(MSR_SPEC_CTRL, bsp_delay_spec_ctrl ? 0 : default_xen_spec_ctrl); + } } static void __init __maybe_unused build_assertions(void) diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h index ec943e1..d36f0e9 100644 --- a/xen/include/asm-x86/spec_ctrl.h +++ b/xen/include/asm-x86/spec_ctrl.h @@ -27,6 +27,8 @@ void init_speculation_mitigations(void); extern bool_t opt_ibpb; + +extern bool_t bsp_delay_spec_ctrl; extern uint8_t default_xen_spec_ctrl; extern uint8_t default_spec_ctrl_flags; -- 2.1.4