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New x86 micro-op vulnerability breaks all known Spectre defenses

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11–20 of 203 posts

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#11

I don't understand this at all; I didn't think the mico-op cache was visible to code written for the x86 ISA at all. Can anyone explain to an idiot (me) how something in micro-op cache can become visible to the outside world?

There is no direct access. These exploits use a side channel attacks. They feed the CPU code where the execution reveals information indirectly.

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#15

There are separate micro op caches per core however they are typically shared among hyperthreads. I wonder if this could be another good reason for cloud vendors to move away from 1vCPU = 1 hyperthread to 1vCPU = 1 core for x86 when sharing machines (not that there weren't enough good reasons already).

Or to roll out more ARM, where there isn't currently any hyperthreading.

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#16

There are separate micro op caches per core however they are typically shared among hyperthreads. I wonder if this could be another good reason for cloud vendors to move away from 1vCPU = 1 hyperthread to 1vCPU = 1 core for x86 when sharing machines (not that there weren't enough good reasons already).

Even putting aside security aspects aside, in general I've been seeing research pop up over the years criticizing SMT's performance claims of ~30%.

Hell, even Amazon's Graviton CPUs don't have it (though I'm sure that's a product of being ARM derived rather than a design decision).

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#17
"In 2018, industry and academic researchers revealed a potentially devastating hardware flaw that made computers and other devices worldwide vulnerable to attack"

Does this apply to all CPU archicetures? The article is a little vague. A fall-out from the monoculture. Hey, where did my post go?

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#18
post #15

There are separate micro op caches per core however they are typically shared among hyperthreads. I wonder if this could be another good reason for cloud vendors to move away from 1vCPU = 1 hyperthread to 1vCPU = 1 core for x86 when sharing machines (not that there weren't enough good reasons already).

Or to roll out more ARM, where there isn't currently any hyperthreading.

Thunder X2 and X3 has 4 way SMT for general purpose, but yes, more ARM is good :-)

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#19

There are separate micro op caches per core however they are typically shared among hyperthreads. I wonder if this could be another good reason for cloud vendors to move away from 1vCPU = 1 hyperthread to 1vCPU = 1 core for x86 when sharing machines (not that there weren't enough good reasons already).

Even putting aside security aspects aside, in general I've been seeing research pop up over the years criticizing SMT's performance claims of ~30%. Hell, even Amazon's Graviton CPUs don't have it (though I'm sure that's a product of being ARM derived rather than a design decision).

ARM vendors must be feeling pretty good about themselves yeah, but if you take AMD's cores... SMT might not be a huge win in every benchmark, but you just can't keep that wide backend fed from a single hyperthread (at least I can't!).

So turning SMT off is at the least wasted potential for those cores, the way they've been designed

Re: New x86 micro-op vulnerability breaks all known Spectre defenses

#20

There are separate micro op caches per core however they are typically shared among hyperthreads. I wonder if this could be another good reason for cloud vendors to move away from 1vCPU = 1 hyperthread to 1vCPU = 1 core for x86 when sharing machines (not that there weren't enough good reasons already).

Why would this be an issue for machines on the cloud? If someone can upload binaries to your machine you have bigger problems, no?
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