linux-kernel - Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected by BHI

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Date: Thu, 11 Apr 2024 23:56:39 +0800
From: Chao Gao <chao.gao@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>
CC: Alexandre Chartre <alexandre.chartre@...cle.com>, Andrew Cooper
	<andrew.cooper3@...rix.com>, <x86@...nel.org>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <daniel.sneddon@...ux.intel.com>,
	<pawan.kumar.gupta@...ux.intel.com>, <tglx@...utronix.de>,
	<konrad.wilk@...cle.com>, <peterz@...radead.org>,
	<gregkh@...uxfoundation.org>, <seanjc@...gle.com>,
	<dave.hansen@...ux.intel.com>, <nik.borisov@...e.com>, <kpsingh@...nel.org>,
	<longman@...hat.com>, <bp@...en8.de>
Subject: Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected
 by BHI

On Thu, Apr 11, 2024 at 05:20:30PM +0200, Paolo Bonzini wrote:
>On Thu, Apr 11, 2024 at 5:13 PM Alexandre Chartre
><alexandre.chartre@...cle.com> wrote:
>> I think that Andrew's concern is that if there is no eIBRS on the host then
>> we do not set X86_BUG_BHI on the host because we know the kernel which is
>> running and this kernel has some mitigations (other than the explicit BHI
>> mitigations) and these mitigations are enough to prevent BHI. But still
>> the cpu is affected by BHI.
>
>Hmm, then I'm confused. It's what I wrote before: "The (Linux or
>otherwise) guest will make its own determinations as to whether BHI
>mitigations are necessary. If the guest uses eIBRS, it will run with
>mitigations" but you said machines without eIBRS are fine.
>
>If instead they are only fine _with Linux_, then yeah we cannot set
>BHI_NO in general. What we can do is define a new bit that is in the
>KVM leaves. The new bit is effectively !eIBRS, except that it is
>defined in such a way that, in a mixed migration pool, both eIBRS and
>the new bit will be 0.

This looks a good solution.

We can also introduce a new bit indicating the effectiveness of the short
BHB-clearing sequence. KVM advertises this bit for all pre-SPR/ADL parts.
Only if the bit is 1, guests will use the short BHB-clearing sequence.
Otherwise guests should use the long sequence. In a mixed migration pool,
the VMM shouldn't expose the bit to guests.

>
>Paolo
>
>