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0 pointsbonzini8y ago0 comments

New CPU microcode is enough, though at a performance price. On pre-Zen AMD there is also a chicken bit to disable indirect branch prediction. (It feels good to be finally able to speak about this freely!!!)

I don't know for which processors Intel and AMD plan to release microcode updates.

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contrarian_8y ago

> New CPU microcode is enough

What would that entail? Disabling speculation completely? Disabling memory accesses during speculation?

bonziniOP8y ago

Disabling indirect branch prediction (and thus speculation after indirect branches) while in kernel mode, or flushing the indirect branch predictor on kernel mode entry. Both need OS support in addition to the microcode, but the change is less invasive than PTI.

contrarian_8y ago

That only fixes one variant of Spectre, and only for code running in kernel mode.

The "out of bounds" Spectre variant is still feasible.

Also: What about hyperthreads? It seems to be many people's assumption that the BTB is shared within a physical core.

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PeterisP8y ago

Wouldn't that be a serious performance hit?

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0 pointsbonzini8y ago0 comments

I don't know for which processors Intel and AMD plan to release microcode updates.

0 comments

contrarian_8y ago

> New CPU microcode is enough

What would that entail? Disabling speculation completely? Disabling memory accesses during speculation?

bonziniOP8y ago

contrarian_8y ago

That only fixes one variant of Spectre, and only for code running in kernel mode.

The "out of bounds" Spectre variant is still feasible.

Also: What about hyperthreads? It seems to be many people's assumption that the BTB is shared within a physical core.

1 more reply

PeterisP8y ago

Wouldn't that be a serious performance hit?

1 more reply

j / k navigate · click thread line to collapse