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Linux page table isolation is not needed on AMD processors

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Re: Linux page table isolation is not needed on AMD processors

#201

Post by amluto »

tfcata wrote: post #191
amluto wrote: post #186

Earlier quoted context omitted.

AMD doesn't exactly do an amazing job of avoiding gotchas in their CPUs. They have a bizarre idea of what writing zero to a segment register should do (resulting in info leaks that were only recently fixed on Linux), their demented leaky IRET is even more demented than Intel's, and their SYSRET's handling of SS is downright nutty. OTOH, Intel's SYSRET is actively dangerous and has resulted in severe security holes, a…

Can you post a few links maybe to the SYSRET issue mentioned? Just curious.

SYSRET on Intel will fault with #GP if the kernel tries to go to a noncanonical user RIP. The #GP comes from kernel mode but with the user RSP. Before SMAP, this was an easy root if it happened. With SMAP, it's still pretty bad. AMD CPUs instead allow SYSRET to succeed and send #PF afterwards, which is very safe.

AMD CPUs are differently dumb. If SYSRET is issued while SS=0, then the SS register ends up in a bogus state in which it appears to contain the correct value but 32-bit stack access fails. Search the Linux kernel for "SYSRET_SS_ATTRS" for the workaround.

Re: Linux page table isolation is not needed on AMD processors

#202

Post by eropple »

manmal wrote: post #126
unethical_ban wrote: post #100

Earlier quoted context omitted.

AMD's triple core processors were quads with disabled cores. Often times processors within a line are processors with manually set lower clock multipliers or disabled cache. Sounds like Intel has just made it unlockable instead of permanent. It just brings to the fore what was already being done, and makes us question again the ethics of pricing models.

Nvidia‘s GeForce cards could be converted to Quadro cards by opening a chip and adding some lines with a pencil. Don’t think that this still works, but a colleague of my father did it for his home PC.

One line, AFAIK. It was just a trace.

However, that (and the later software modification) could both hamper performance in games and could exhibit correctness problems in accuracy-focused use cases, so it was rarely a great idea.

Re: Linux page table isolation is not needed on AMD processors

#203

Post by dingo_bat »

rdudek wrote: post #89
kevin_thibedeau wrote: post #42

Earlier quoted context omitted.

Soon, malicious JavaScript will be able to own your computer.

Don't most of them do that already?

Right now, they just own you and your data. With this, they will own your machine.

Re: Linux page table isolation is not needed on AMD processors

#204

Post by bhouston »

electic wrote: post #170

This is going to have dramatic effect on the cloud computing market. It might make sense to make sure any VMs you run are on AMD processors or it can really hurt your performance and basically cost you more to do the same workload. It also seems, from early benchmarks, this can slaughter performance with databases.

I wonder if cloud providers will ask Intel for partial refunds when their CPUs get 5% to 30% slower than promised?

Re: Linux page table isolation is not needed on AMD processors

#205

Post by bhouston »

artellectual wrote: post #196

Essentially looks like Intel compromised (whether intentional or not is a different point) the design to get the speed boost that gave them the lead over AMD for the past decade. Will be interesting to see how all this plays out.

What chip exactly introduced this feature?

Core 2 architecture? Nehalem?

Re: Linux page table isolation is not needed on AMD processors

#206

Post by phamilton »

freeone3000 wrote: post #76
dboreham wrote: post #53

Earlier quoted context omitted.

I think it will because it shows the downside of a monoculture. Hence big purchasers of CPUs will want to diversify. Also good for ARM vendors I suppose. Disclosure : bought AMD this morning before headlines saying "Buy AMD, short INTC" appeared.

Why would this cause you to diversify? Long-term negative effects of a monoculture are not evenly distributed to purchasers. In fact, if you ran both AMD and Intel CPUs, you'd see application performance differences solely based on processor architecture. This makes application deployment planning way harder. At any given time, there's one CPU that should be purchased, and artificially introducing two "so they don't…

Imagine you are AWS. You have a range of instance types with various performance characteristics. Having customers move from c5s to c4s is much better than customers moving from AWS to GCE.

Re: Linux page table isolation is not needed on AMD processors

#207

Post by vasili111 »

electic wrote: post #170

This is going to have dramatic effect on the cloud computing market. It might make sense to make sure any VMs you run are on AMD processors or it can really hurt your performance and basically cost you more to do the same workload. It also seems, from early benchmarks, this can slaughter performance with databases.

Don't worry. I don't think that there will be two separate kernels for Intel and AMD. I think performance drop will be on both CPUs no matter has it the bug or not.

Re: Linux page table isolation is not needed on AMD processors

#208

Post by russdill »

vasili111 wrote: post #207
electic wrote: post #170

This is going to have dramatic effect on the cloud computing market. It might make sense to make sure any VMs you run are on AMD processors or it can really hurt your performance and basically cost you more to do the same workload. It also seems, from early benchmarks, this can slaughter performance with databases.

Don't worry. I don't think that there will be two separate kernels for Intel and AMD. I think performance drop will be on both CPUs no matter has it the bug or not.

You can turn it off and on via a commandline flag.

Re: Linux page table isolation is not needed on AMD processors

#210

Post by kbart »

vasili111 wrote: post #207
electic wrote: post #170

This is going to have dramatic effect on the cloud computing market. It might make sense to make sure any VMs you run are on AMD processors or it can really hurt your performance and basically cost you more to do the same workload. It also seems, from early benchmarks, this can slaughter performance with databases.

Don't worry. I don't think that there will be two separate kernels for Intel and AMD. I think performance drop will be on both CPUs no matter has it the bug or not.

No. The check is being made what CPU is underlying before applying the fix.
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