linux-kernel - Re: [PATCH] x86/tsx: clear RTM and HLE when MSR_IA32_TSX

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Date:   Mon, 6 Sep 2021 22:28:01 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Hao Peng <flyingpenghao@...il.com>, Borislav Petkov <bp@...en8.de>,
        tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/tsx: clear RTM and HLE when MSR_IA32_TSX_CTRL is not
 supported

On 07.09.2021 10:56, Xiaoyao Li wrote:
>On 9/7/2021 10:35 AM, Hao Peng wrote:
>>On Tue, Sep 7, 2021 at 10:08 AM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
>>>
>>>On 9/7/2021 9:47 AM, Hao Peng wrote:
>>>>On Mon, Sep 6, 2021 at 5:30 PM Borislav Petkov <bp@...en8.de> wrote:
>>>>>
>>>>>On Mon, Sep 06, 2021 at 10:46:05AM +0800, Hao Peng wrote:
>>>>>>If hypervisor does not support MSR_IA32_TSX_CTRL, but guest supports
>>>>>>RTM and HLE features, it will affect TAA mitigation.
>>>>>>
>>>>>>Signed-off-by: Peng Hao <flyingpeng@...cent.com>
>>>>>>---
>>>>>>   arch/x86/kernel/cpu/tsx.c | 7 +++++++
>>>>>>   1 file changed, 7 insertions(+)
>>>>>>
>>>>>>diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
>>>>>>index 9c7a5f049292..5e852c14fef2 100644
>>>>>>--- a/arch/x86/kernel/cpu/tsx.c
>>>>>>+++ b/arch/x86/kernel/cpu/tsx.c
>>>>>>@@ -122,6 +122,13 @@ void __init tsx_init(void)
>>>>>>
>>>>>>          if (!tsx_ctrl_is_supported()) {
>>>>>>                  tsx_ctrl_state = TSX_CTRL_NOT_SUPPORTED;
>>>>>>+
>>>>>>+               /* If hypervisor does not support MSR_IA32_TSX_CTRL emulation,
>>>>>>+                * but guest supports RTM and HLE features, it will affect TAA
>>>>>>+                * （tsx_async_abort）mitigation.
>>>>>>+                */
>>>>>>+               setup_clear_cpu_cap(X86_FEATURE_RTM);
>>>>>>+               setup_clear_cpu_cap(X86_FEATURE_HLE);
>>>
>>>anyway, IMHO, we shouldn't do anything here for TAA. It should be in
>>>taa_select_mitigation()
>>>
>>>>>>                  return;
>>>>>>          }
>>>>>
>>>>>How does that even happen - the hypervisor does not support the MSR but
>>>>>"guest supports" TSX features?!
>>>>>
>>>>>I guess the guest is detecting it wrong.
>>>>>
>>>>>What hypervisor, what guest, how do I reproduce?
>>>>>
>>>>hypervisor is kvm, guest is linux too.
>>>>>Please give full details.
>>>>>
>>>>The host I used is kernel-5.4, and guest is kernel-5.13.
>>>>MSR_IA32_TSX_CTRL is exposed
>>>>to guest and guest to support RTM and HLE features, no direct
>>>>dependence. at the qemu I
>>>>started guest with -cpu host-model.
>>>>I have viewed the code of kernel-5.4, and MSR_IA32_TSX_CTRL is not
>>>>exposed to guest.
>>>
>>>Does guest see TAA_NO bit?
>>>
>>Guest can't see taa_no, which requires updating qemu to solve. But I think
>>there is a compatibility process here.
>
>Anyway, there should be some existing code in kernel already to handle 
>the case that CPUID reports TRM while MSR_IA32_CORE_CAPABILITIES 
>doesn't report MSR_TSX_CTRL nor TAA_NO.

When TSX_CTRL MSR is not supported, TAA mitigation is same as MDS, and
it is deployed automatically provided the system has a recent microcode.

Thanks,
Pawan