Re: [Xen-devel] [PATCH v3 11/28] xen/x86: Clear dependent features when clearing a cpu cap

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/03/16 19:51, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 15, 2016 at 03:35:07PM +0000, Andrew Cooper wrote:
>> When clearing a cpu cap, clear all dependent features.  This avoids having a
>> featureset with intermediate features disabled, but leaf features enabled.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>
>> v3:
>>  * Style fixes.  Use __test_and_set_bit()
>> ---
>>  xen/arch/x86/cpu/common.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
>> index cd168c8..cb1cab3 100644
>> --- a/xen/arch/x86/cpu/common.c
>> +++ b/xen/arch/x86/cpu/common.c
>> @@ -53,8 +53,22 @@ static unsigned int cleared_caps[NCAPINTS];
>>  
>>  void __init setup_clear_cpu_cap(unsigned int cap)
>>  {
>> +    const uint32_t *dfs;
>> +    unsigned int i;
>> +
>> +    if (__test_and_set_bit(cap, cleared_caps))
>> +            return;
>> +
>>      __clear_bit(cap, boot_cpu_data.x86_capability);
>> -    __set_bit(cap, cleared_caps);
>> +    dfs = lookup_deep_deps(cap);
>> +
>> +    if (!dfs)
>> +            return;
>> +
>> +    for (i = 0; i < FSCAPINTS; ++i) {
>> +            cleared_caps[i] |= dfs[i];
>> +            boot_cpu_data.x86_capability[i] &= ~dfs[i];
>> +    }
> That is OK, but what if the feature we clear has a dependeny
> chain - of clearing other features?

The purpose of patch 10 "xen/x86: Generate deep dependencies of
features" is to flatten the dependency tree into a single bitmap, so
dfs[] in this case is the complete set of all eventual features
dependent on cap.

~Andrew

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