Messages in this thread Patch in this message | | | Date | Thu, 24 Aug 2023 15:04:40 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 02/22] x86/srso: Set CPUID feature bits independently of bug or mitigation status |
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On Thu, Aug 24, 2023 at 06:24:20AM +0200, Borislav Petkov wrote: > On Wed, Aug 23, 2023 at 01:22:34PM +0100, Andrew Cooper wrote: > > In my previous reply, I explained why this goes wrong when Linux ignores > > the CPUID bit provided by the hypervisor and decides to probe manually. > > Send a patch and explain in its commit message *why* this is needed. > > > No. > > Hell yeah! > > How do you expect us to support use cases we don't know about?! > > > You don't get to take my code, break it when integrating it into Linux, > > then dismiss the bug as something hypothetical that you don't want to fix. > > I have no clue what you're talking about but it sounds like > a misunderstanding. All I'm saying is, the live migration use cases the > kernel should support, should be documented first. If there's no > documentation for them, *then* you have hypothetical. > > So patches explaining what we're supporting are welcome.
Something like this?
From: Josh Poimboeuf <jpoimboe@kernel.org> Subject: [PATCH] x86/srso: Don't probe microcode in a guest
To support live migration, the hypervisor sets the "lowest common denominator" of features. Probing the microcode isn't allowed because any detected features might go away after a migration.
As Andy Cooper states:
"Linux must not probe microcode when virtualised. What it may see instantaneously on boot (owing to MSR_PRED_CMD being fully passed through) is not accurate for the lifetime of the VM."
Rely on the hypervisor to set the needed IBPB_BRTYPE and SBPB bits.
Fixes: 1b5277c0ea0b ("x86/srso: Add SRSO_NO support") Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org> --- arch/x86/kernel/cpu/amd.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index b08af929135d..28e77c5d6484 100644 --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -767,7 +767,7 @@ static void early_init_amd(struct cpuinfo_x86 *c) if (cpu_has(c, X86_FEATURE_TOPOEXT)) smp_num_siblings = ((cpuid_ebx(0x8000001e) >> 8) & 0xff) + 1; - if (!cpu_has(c, X86_FEATURE_IBPB_BRTYPE)) { + if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && !cpu_has(c, X86_FEATURE_IBPB_BRTYPE)) { if (c->x86 == 0x17 && boot_cpu_has(X86_FEATURE_AMD_IBPB)) setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE); else if (c->x86 >= 0x19 && !wrmsrl_safe(MSR_IA32_PRED_CMD, PRED_CMD_SBPB)) { -- 2.41.0
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