Adding [email protected] to CC so this regression is tracked.
Regards,
Tomeu
On 8 November 2017 at 09:37, Tomeu Vizoso <[email protected]> wrote:
> On 6 November 2017 at 23:01, Tom Lendacky <[email protected]> wrote:
>> On 11/6/2017 3:41 PM, H. Peter Anvin wrote:
>>>
>>> On 11/06/17 12:17, Tom Lendacky wrote:
>>>>
>>>> When crosvm is used to boot a kernel as a VM, the SMP MP-table is found
>>>> at physical address 0x0. This causes mpf_base to be set to 0 and a
>>>> subsequent "if (!mpf_base)" check in default_get_smp_config() results in
>>>> the MP-table not being parsed. Further into the boot this results in an
>>>> oops when attempting a read_apic_id().
>>>>
>>>> Add a boolean variable that is set to true when the MP-table is found.
>>>> Use this variable for testing if the MP-table was found so that even a
>>>> value of 0 for mpf_base will result in continued parsing of the MP-table.
>>>>
>>>> Reported-by: Tomeu Vizoso <[email protected]>
>>>> Signed-off-by: Tom Lendacky <[email protected]>
>>>
>>>
>>> Ahem... did anyone ever tell you that this is an epicly bad idea on your
>>> part? The low megabyte of physical memory has very special meaning on
>>> x86, and deviating from the standard use of this memory is a *very*
>>> dangerous thing to do, and imposing on the kernel a "fake null pointer"
>>> requirement that exists only for the convenience of your particular
>>> brokenness is not okay.
>>>
>>> -hpa
>>
>>
>> That was my initial thought... what was something doing down at the start
>> of memory. But when I looked at default_find_smp_config() it specifically
>> scans the bottom 1K for a an MP-table signature. I was hoping to get some
>> feedback as to whether this would really be an acceptable thing to do. So
>> I'm good with this patch being rejected, but the change I made in
>>
>> 5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data")
>>
>> does break something that was working before.
>
> Do I understand correctly that the best we can do right now is
> reverting 5997efb96756?
>
> Thanks,
>
> Tomeu
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And now with the correct email.
Sorry about that,
Tomeu
On 16 November 2017 at 10:16, Tomeu Vizoso <[email protected]> wrote:
> Adding [email protected] to CC so this regression is tracked.
>
> Regards,
>
> Tomeu
>
> On 8 November 2017 at 09:37, Tomeu Vizoso <[email protected]> wrote:
>> On 6 November 2017 at 23:01, Tom Lendacky <[email protected]> wrote:
>>> On 11/6/2017 3:41 PM, H. Peter Anvin wrote:
>>>>
>>>> On 11/06/17 12:17, Tom Lendacky wrote:
>>>>>
>>>>> When crosvm is used to boot a kernel as a VM, the SMP MP-table is found
>>>>> at physical address 0x0. This causes mpf_base to be set to 0 and a
>>>>> subsequent "if (!mpf_base)" check in default_get_smp_config() results in
>>>>> the MP-table not being parsed. Further into the boot this results in an
>>>>> oops when attempting a read_apic_id().
>>>>>
>>>>> Add a boolean variable that is set to true when the MP-table is found.
>>>>> Use this variable for testing if the MP-table was found so that even a
>>>>> value of 0 for mpf_base will result in continued parsing of the MP-table.
>>>>>
>>>>> Reported-by: Tomeu Vizoso <[email protected]>
>>>>> Signed-off-by: Tom Lendacky <[email protected]>
>>>>
>>>>
>>>> Ahem... did anyone ever tell you that this is an epicly bad idea on your
>>>> part? The low megabyte of physical memory has very special meaning on
>>>> x86, and deviating from the standard use of this memory is a *very*
>>>> dangerous thing to do, and imposing on the kernel a "fake null pointer"
>>>> requirement that exists only for the convenience of your particular
>>>> brokenness is not okay.
>>>>
>>>> -hpa
>>>
>>>
>>> That was my initial thought... what was something doing down at the start
>>> of memory. But when I looked at default_find_smp_config() it specifically
>>> scans the bottom 1K for a an MP-table signature. I was hoping to get some
>>> feedback as to whether this would really be an acceptable thing to do. So
>>> I'm good with this patch being rejected, but the change I made in
>>>
>>> 5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data")
>>>
>>> does break something that was working before.
>>
>> Do I understand correctly that the best we can do right now is
>> reverting 5997efb96756?
>>
>> Thanks,
>>
>> Tomeu
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