[MODERATED] Re: [PATCH v7 00/10] TAAv7 0

On 21/10/2019 22:32, speck for Andy Lutomirski wrote:
> On 10/21/19 1:22 PM, speck for Pawan Gupta wrote:
>> From: Pawan Gupta <[email protected]>
>> Subject: [PATCH v7 00/10] TAAv7
>>
>> Changes since v6:
>> - Add Michal's patch to allow tsx=on|off|auto via CONFIG
>> - Rebase to v5.4-rc4
>> - Changelog, comments and documentation update.
>>
>> Changes since v5:
>> - Remove unsafe X86_FEATURE_RTM toggles.
> I'm wondering if maybe these patches shouldn't touch the cpu
> capabilities at all.  After all, even with TSX toggled off, TSX is still
> present -- XBEGIN doesn't give #UD.

So, there is some history here.

The first microcode version had XBEGIN giving #UD, and a fair chunk of
ancillary behaviour revolved around that.

However, that is rather catastrophic for late-loading the microcode and
securing the system in place, so the behaviour was relaxed from #UD to
abort.

That way, folk who late-load the microcode can secure their systems by
setting the RTM_ABORT bit, and folk booting "fresh" with the new
microcode can set both the RTM_ABORT and CPUID bits, to cause userspace
to believe there is no RTM at all, and avoid their unconditional abort.

~Andrew

P.S. I did ask if we could have both #UD and abort, and got a firm no
based on patch space.