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Subject: Re: [PATCH v2 4/5] prctl: Hook L1D flushing in via prctl
To:     Tom Lendacky <thomas.lendacky@amd.com>, <tglx@linutronix.de>,
        <linux-kernel@vger.kernel.org>
CC:     <jpoimboe@redhat.com>, <tony.luck@intel.com>,
        <keescook@chromium.org>, <benh@kernel.crashing.org>,
        <x86@kernel.org>, <dave.hansen@intel.com>,
        <torvalds@linux-foundation.org>, <mingo@kernel.org>
References: <20200729001103.6450-1-sblbir@amazon.com>
 <20200729001103.6450-5-sblbir@amazon.com>
 <982c1d40-aac1-df0c-c3b7-2699dc0b9b6f@amd.com>
From:   "Singh, Balbir" <sblbir@amazon.com>
Message-ID: <33b99d83-f2de-6984-c3d6-a44960aac828@amazon.com>
Date:   Thu, 30 Jul 2020 10:13:09 +1000
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On 29/7/20 11:14 pm, Tom Lendacky wrote:
> 
> 
> On 7/28/20 7:11 PM, Balbir Singh wrote:
>> Use the existing PR_GET/SET_SPECULATION_CTRL API to expose the L1D
>> flush capability. For L1D flushing PR_SPEC_FORCE_DISABLE and
>> PR_SPEC_DISABLE_NOEXEC are not supported.
>>
>> There is also no seccomp integration for the feature.
>>
>> Signed-off-by: Balbir Singh <sblbir@amazon.com>
>> ---
>>  arch/x86/kernel/cpu/bugs.c | 54 ++++++++++++++++++++++++++++++++++++++
>>  arch/x86/mm/tlb.c          | 25 +++++++++++++++++-
>>  include/uapi/linux/prctl.h |  1 +
>>  3 files changed, 79 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> index 0b71970d2d3d..935ea88313ab 100644
>> --- a/arch/x86/kernel/cpu/bugs.c
>> +++ b/arch/x86/kernel/cpu/bugs.c
>> @@ -295,6 +295,13 @@ enum taa_mitigations {
>>       TAA_MITIGATION_TSX_DISABLED,
>>  };
>>
>> +enum l1d_flush_out_mitigations {
>> +     L1D_FLUSH_OUT_OFF,
>> +     L1D_FLUSH_OUT_ON,
>> +};
>> +
>> +static enum l1d_flush_out_mitigations l1d_flush_out_mitigation __ro_after_init = L1D_FLUSH_OUT_ON;
>> +
>>  /* Default mitigation for TAA-affected CPUs */
>>  static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;
>>  static bool taa_nosmt __ro_after_init;
>> @@ -378,6 +385,18 @@ static void __init taa_select_mitigation(void)
>>       pr_info("%s\n", taa_strings[taa_mitigation]);
>>  }
>>
>> +static int __init l1d_flush_out_parse_cmdline(char *str)
>> +{
>> +     if (!boot_cpu_has_bug(X86_BUG_L1TF))
>> +             return 0;
> 
> Shouldn't this set the l1d_flush_out_mitigation to L1D_FLUSH_OUT_OFF since
> it is set to L1D_FLUSH_OUT_ON by default? Or does it not matter because
> the enable_l1d_flush_for_task() will return -EINVAL if the cpu doesn't
> have the L1TF bug?
>
> I guess it depends on what you want l1d_flush_out_prctl_set() and
> l1d_flush_out_prctl_get() to return in this case.
> 

Exactly! We want to differentiate between force disabled and not applicable.


Thanks for the review,
Balbir Singh.