Re: [Xen-devel] [PATCH 2/3] x86: suppress BTI mitigations around S3 suspend/resume

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

Andrew Cooper:
> On 13/04/18 19:25, Simon Gaiser wrote:
>> Jan Beulich:
>>> NMI and #MC can occur at any time after S3 resume, yet the MSR_SPEC_CTRL
>>> may become available only once we're reloaded microcode. Make
>>> SPEC_CTRL_ENTRY_FROM_INTR_IST and DO_SPEC_CTRL_EXIT_TO_XEN no-ops for
>>> the critical period of time.
>>>
>>> Also set the MSR back to its intended value.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> --- a/xen/arch/x86/acpi/power.c
>>> +++ b/xen/arch/x86/acpi/power.c
>>> @@ -28,6 +28,7 @@
>>>  #include <asm/tboot.h>
>>>  #include <asm/apic.h>
>>>  #include <asm/io_apic.h>
>>> +#include <asm/spec_ctrl.h>
>>>  #include <acpi/cpufreq/cpufreq.h>
>>>  
>>>  uint32_t system_reset_counter = 1;
>>> @@ -163,6 +164,7 @@ static int enter_state(u32 state)
>>>  {
>>>      unsigned long flags;
>>>      int error;
>>> +    struct cpu_info *ci;
>>>      unsigned long cr4;
>>>  
>>>      if ( (state <= ACPI_STATE_S0) || (state > ACPI_S_STATES_MAX) )
>>> @@ -210,6 +212,10 @@ static int enter_state(u32 state)
>>>      else
>>>          error = 0;
>>>  
>>> +    ci = get_cpu_info();
>>> +    ci->use_shadow_spec_ctrl = 0;
>>> +    ci->bti_ist_info = 0;
>>> +
>>>      ACPI_FLUSH_CPU_CACHE();
>>>  
>>>      switch ( state )
>>> @@ -248,6 +254,11 @@ static int enter_state(u32 state)
>>>  
>>>      microcode_resume_cpu(0);
>>>  
>>> +    ci->bti_ist_info = default_bti_ist_info;
>>> +    asm volatile (ALTERNATIVE("", "wrmsr", X86_FEATURE_XEN_IBRS_SET)
>> This does not compile for me:
>>
>>   power.c: Assembler messages:
>>   power.c:272: Error: value of 257 too large for field of 1 bytes at 0
>>
>> Changing the alternative based on the other "wrmsr" calls fixes it:
>>
>>   asm volatile (ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_XEN_IBRS_SET)
> 
> Ah - you're presumably back-porting this to 4.8?
> 
> Jan's code is correct for staging, and your version here is correct for
> all currently-released versions of Xen.  (I've done quite a lot of
> playing with alternatives generation for 4.11.)

Yeah, sorry, I should have checked if it works with staging.

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