Re: [Xen-devel] [PATCH v8 03/16] microcode/intel: extend microcode_update_match()

[Chao Gao <chao.gao@xxxxxxxxx>]

On Mon, Aug 05, 2019 at 09:27:58AM +0000, Jan Beulich wrote:
>On 05.08.2019 07:58, Chao Gao wrote:
>> On Fri, Aug 02, 2019 at 01:29:14PM +0000, Jan Beulich wrote:
>>> On 01.08.2019 12:22, Chao Gao wrote:
>>>> --- a/xen/arch/x86/microcode_intel.c
>>>> +++ b/xen/arch/x86/microcode_intel.c
>>>> @@ -134,14 +134,35 @@ static int collect_cpu_info(unsigned int cpu_num, 
>>>> struct cpu_signature *csig)
>>>>        return 0;
>>>>    }
>>>>    
>>>> -static inline int microcode_update_match(
>>>> -    unsigned int cpu_num, const struct microcode_header_intel *mc_header,
>>>> -    int sig, int pf)
>>>> +static enum microcode_match_result microcode_update_match(
>>>> +    const struct microcode_header_intel *mc_header, unsigned int sig,
>>>> +    unsigned int pf, unsigned int rev)
>>>>    {
>>>> -    struct ucode_cpu_info *uci = &per_cpu(ucode_cpu_info, cpu_num);
>>>> -
>>>> -    return (sigmatch(sig, uci->cpu_sig.sig, pf, uci->cpu_sig.pf) &&
>>>> -            (mc_header->rev > uci->cpu_sig.rev));
>>>> +    const struct extended_sigtable *ext_header;
>>>> +    const struct extended_signature *ext_sig;
>>>> +    unsigned long data_size = get_datasize(mc_header);
>>>> +    unsigned int i;
>>>> +    const void *end = (const void *)mc_header + get_totalsize(mc_header);
>>>> +
>>>> +    if ( sigmatch(sig, mc_header->sig, pf, mc_header->pf) )
>>>> +        return (mc_header->rev > rev) ? NEW_UCODE : OLD_UCODE;
>>>
>>> Both here and ...
>>>
>>>> +    ext_header = (const void *)(mc_header + 1) + data_size;
>>>> +    ext_sig = (const void *)(ext_header + 1);
>>>> +
>>>> +    /*
>>>> +     * Make sure there is enough space to hold an extended header and 
>>>> enough
>>>> +     * array elements.
>>>> +     */
>>>> +    if ( (end < (const void *)ext_sig) ||
>>>> +         (end < (const void *)(ext_sig + ext_header->count)) )
>>>> +        return MIS_UCODE;
>>>> +
>>>> +    for ( i = 0; i < ext_header->count; i++ )
>>>> +        if ( sigmatch(sig, ext_sig[i].sig, pf, ext_sig[i].pf) )
>>>> +            return (mc_header->rev > rev) ? NEW_UCODE : OLD_UCODE;
>>>
>>> ... here there's again an assumption that there's strict ordering
>>> between blobs, but as mentioned in reply to a later patch of an
>>> earlier version this isn't the case. Therefore the function can't
>>> be used to compare two arbitrary blobs, it may only be used to
>>> compare a blob with what is already loaded into a CPU. I think it
>>> is quite important to mention this restriction in a comment ahead
>>> of the function.
>>>
>>> The code itself looks fine to me, and a comment could perhaps be
>>> added while committing; with such a comment
>> 
>> Agree. Because there will be a version 9, I can add a comment.
>
>Having seen the uses in later patches, I think a comment is not the
>way to go. Instead I think you want to first match _both_
>signatures against the local CPU (unless e.g. for either side this
>is logically guaranteed),

Yes. It is guaranteed at the first place: we ignore any patch that
doesn't match with the CPU signature when parsing the ucode blob.
  
>and return DIS_UCODE upon mismatch. Only
>then should you actually compare the two signatures. While from a
>pure, abstract patch ordering perspective this isn't correct, we
>only care about patches applicable to the local CPU anyway, and for
>that case the extra restriction is fine. This way you'll be able to
>avoid taking extra precautions in vendor-independent code just to
>accommodate an Intel specific requirement.

Yes. I agree and it seems that no further change is needed except
the implementation of ->compare_patch. Please correct me if I am wrong.

Thanks
Chao



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