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> Intel CPUs allow this, but AMD CPUs do not. Do we know this for sure ? I see now that the meltdown paper has a suggestion in chapter 7.1 that the CPU should
by noselasd 9y ago
> Intel CPUs allow this, but AMD CPUs do not.
Do we know this for sure ? I see now that the meltdown paper has a suggestion in chapter 7.1 that the CPU should perform the permission check on the page table before data is fetched to the cache,
But dismisses that suggestion on the grounds it will haves very high performance impact.
Are you saying AMD does the check first anyway, and either takes the performance hit or has found a way to not take a performance hit ?
- kimdcmason 9y agoMy understanding isn't especially deep, but I believe AMD prevents access to unloaded pages that the code doesn't have permission to access, even for speculative access. Their specific quote is: "AMD processors are not subject to the types of attacks that the kernel page table isolation feature protects against. The AMD microarchitecture does not allow memory references, including speculative references, that access higher privileged data when running in a lesser privileged mode when that access would result in a page fault."