On 14.06.2023 19:25, Andrew Cooper wrote: > On 13/06/2023 10:30 am, Jan Beulich wrote: >> On 12.06.2023 18:13, Andrew Cooper wrote: >>> @@ -593,15 +596,93 @@ static bool __init retpoline_calculations(void) >>> return false; >>> >>> /* >>> - * RSBA may be set by a hypervisor to indicate that we may move to a >>> - * processor which isn't retpoline-safe. >>> + * The meaning of the RSBA and RRSBA bits have evolved over time. The >>> + * agreed upon meaning at the time of writing (May 2023) is thus: >>> + * >>> + * - RSBA (RSB Alternative) means that an RSB may fall back to an >>> + * alternative predictor on underflow. Skylake uarch and later all >>> have >>> + * this property. Broadwell too, when running microcode versions >>> prior >>> + * to Jan 2018. >>> + * >>> + * - All eIBRS-capable processors suffer RSBA, but eIBRS also >>> introduces >>> + * tagging of predictions with the mode in which they were learned. >>> So >>> + * when eIBRS is active, RSBA becomes RRSBA (Restricted RSBA). >>> + * >>> + * - CPUs are not expected to enumerate both RSBA and RRSBA. >>> + * >>> + * Some parts (Broadwell) are not expected to ever enumerate this >>> + * behaviour directly. Other parts have differing enumeration with >>> + * microcode version. Fix up Xen's idea, so we can advertise them >>> safely >>> + * to guests, and so toolstacks can level a VM safety for migration. >>> + * >>> + * The following states exist: >>> + * >>> + * | | RSBA | EIBRS | RRSBA | Notes | Action | >>> + * |---+------+-------+-------+--------------------+---------------| >>> + * | 1 | 0 | 0 | 0 | OK (older parts) | Maybe +RSBA | >>> + * | 2 | 0 | 0 | 1 | Broken | +RSBA, -RRSBA | >>> + * | 3 | 0 | 1 | 0 | OK (pre-Aug ucode) | +RRSBA | >>> + * | 4 | 0 | 1 | 1 | OK | | >>> + * | 5 | 1 | 0 | 0 | OK | | >>> + * | 6 | 1 | 0 | 1 | Broken | -RRSBA | >>> + * | 7 | 1 | 1 | 0 | Broken | -RSBA, +RRSBA | >>> + * | 8 | 1 | 1 | 1 | Broken | -RSBA | >> You've kept the Action column as you had it originally, despite no longer >> applying all the fixups. Wouldn't it make sense to mark those we don't do, >> e.g. by enclosing in parentheses? > > Hmm, yes. How does this look? > > | | RSBA | EIBRS | RRSBA | Notes | Action (in principle) | > |---+------+-------+-------+--------------------+-----------------------| > | 1 | 0 | 0 | 0 | OK (older parts) | Maybe +RSBA | > | 2 | 0 | 0 | 1 | Broken | (+RSBA, -RRSBA) | > | 3 | 0 | 1 | 0 | OK (pre-Aug ucode) | +RRSBA | > | 4 | 0 | 1 | 1 | OK | | > | 5 | 1 | 0 | 0 | OK | | > | 6 | 1 | 0 | 1 | Broken | (-RRSBA) | > | 7 | 1 | 1 | 0 | Broken | (-RSBA, +RRSBA) | > | 8 | 1 | 1 | 1 | Broken | (-RSBA) |
Yes, I think it's better to have it this way, thanks. Jan
