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

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Message-ID: <d47dcc77-3c8b-4f78-954a-a64d3a905224@citrix.com>
Date: Thu, 11 Apr 2024 10:38:10 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Alexandre Chartre <alexandre.chartre@...cle.com>, x86@...nel.org,
 kvm@...r.kernel.org
Cc: 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, pbonzini@...hat.com
Subject: Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected
 by BHI

On 11/04/2024 10:33 am, Alexandre Chartre wrote:
>
>
> On 4/11/24 10:43, Andrew Cooper wrote:
>> On 11/04/2024 8:24 am, Alexandre Chartre wrote:
>>> When a system is not affected by the BHI bug then KVM should
>>> configure guests with BHI_NO to ensure they won't enable any
>>> BHI mitigation.
>>>
>>> Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
>>> ---
>>>   arch/x86/kvm/x86.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 984ea2089efc..f43d3c15a6b7 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -1678,6 +1678,9 @@ static u64 kvm_get_arch_capabilities(void)
>>>       if (!boot_cpu_has_bug(X86_BUG_GDS) || gds_ucode_mitigated())
>>>           data |= ARCH_CAP_GDS_NO;
>>>   +    if (!boot_cpu_has_bug(X86_BUG_BHI))
>>> +        data |= ARCH_CAP_BHI_NO;
>>
>> This isn't true or safe.
>>
>> Linux only sets X86_BUG_BHI on a subset of affected parts.
>>
>> Skylake for example *is* affected by BHI.  It's just that existing
>> mitigations are believed to suffice to mitigate BHI too.
>>
>> "you happen to be safe if you're doing something else too" doesn't
>> remotely have the same meaning as "hardware doesn't have a history based
>> predictor".
>>
>
> So you mean we can't set ARCH_CAP_BHI_NO for the guest because we
> don't know
> if the guest will run the (other) existing mitigations which are
> believed to
> suffice to mitigate BHI?

Correct.

Also, when a VM really is migrating between different CPUs, things get
far more complicated.

>
> The problem is that we can end up with a guest running extra BHI
> mitigations
> while this is not needed. Could we inform the guest that eIBRS is not
> available
> on the system so a Linux guest doesn't run with extra BHI mitigations?

Well, that's why Intel specified some MSRs at 0x5000xxxx.

Except I don't know anyone currently interested in implementing them,
and I'm still not sure if they work correctly for some of the more
complicated migration cases.

~Andrew