What is tlb bug

Linux: insufficient shootdown for paging-structure caches link. This blog post focuses on the last bug in the list. By the way: Note that the gVisor bug is in memory management code written in Go, which is memory-safe -ish. This demonstrates that in operating system code, "logic bugs" in some places, like page table management, can have consequences that are as severe as those of classical memory safety issues, and are not in the scope of the language's safety guarantees.

Of course, memory-safe languages are still highly useful because they should prevent bugs in random, non-critical pieces of kernel code from corrupting completely unrelated system state, and they allow reviewers to spend more time on the security-critical parts of the system.

If you know what a TLB is, what a TLB flush is, what paging-structure caches are, and how paging-structure caches are managed, you can skip this section. Page tables contain information on how virtual addresses map to physical ones. Page tables are stored in memory, so they are comparatively slow to access; to make address translation fast, CPUs use caches.

The classic caches for this are called Translation Lookaside Buffers TLBs ; they cache mappings from virtual to physical page addresses including mappings for huge pages , or in other words, they more or less cache last-level page table entries. TLB parameters are usually fairly well-documented; for example:. Paging-structure caches are usually less well-documented; but there is official documentation about their existence and necessary precautions when dealing with them.

Paging-structure caches store copies of non-last-level page table entries; they are used when a virtual address without a corresponding TLB entry is being accessed, and they reduce the number of memory accesses for a page table walk. There are some reverse-engineered details about the paging-structure caches of various processors in a VUSec paper in Table 1. It generally has to be assumed that entries in TLBs and paging-structure caches can be evicted by the processor whenever it wants to.

Similarly, it has to be assumed that a processor can create entries in TLBs and paging-structure caches from page table entries whenever it wants to, because memory accesses in speculatively executed code can create such entries. Mechanisms to invalidate TLB entries and paging-structure caches differ between processor architectures:.

Invalidating the TLB entry for a virtual address also at least implies invalidation of any paging-structure cache entries that could be used for translating that virtual address. The ARM architecture provides magic instructions that can perform cross-core TLB invalidation for you; however, if you also need to synchronize against page table walks implemented in software like the Linux kernel , you may have to send IPIs anyway depending on the synchronization mechanism used for page table walks.

The general code pattern for performing cache invalidations for page table entries is:. Remove an entry from a page table, but keep holding a reference to the physical page it points to. Perform a TLB flush either for a specific address, or for the entire address space across all cores that might be using the same page tables as the current thread.

Drop the reference that was held on the physical page, potentially freeing it. This pattern is the same both when unmapping normal data pages and when removing page tables. It can often be batched for better performance - first remove multiple page table entries, then do one TLB flush across cores, then drop all the page references -, but for the mapping of an individual page including page tables , this pattern is generally true. On X86 but ARM64 is similar , there are two bits in a last-level PTE which the CPU can write into as part of address translation: The Accessed bit specifies whether the CPU has ever used the page table entry for address translation; in other words, if the Accessed bit is unset, the value of the page table entry has not been cached by the TLB since the last time the page table entry was written by software.

The Dirty bit specifies whether the CPU has ever used the page table entry for a writing memory access; in other words, if the Dirty bit is unset, no TLB entries that can be used to write to the physical page have been created since the last software write to the PTE.

However, some other types of page table access e. The mremap syscall allows userspace to move a VMA and its associated page table entries. Take page table locks on the old and new page tables. Do a TLB flush if the direct reclaim path is in the middle of stealing some pages from the current process.

Previous Next Sort by votes. Nov 24, 62 0 10, 0. Fun fact AMD lied on fixing the tlb bug with the B3 stepping phenoms. A month ago my uncle made a few cheap pc's for the purpouse of using virtual maschines on them. AMD lied damn it. Oct 17, 3, 7 24, Since I don't see anything saying that your uncle did any testing, I would say there's no way he can blame the CPUs yet Jul 25, 44, 2 , 6, Unfortunately Phenom still suffers from low clock speeds and that's something AMD will be working on in the coming months.

It will take a combination of higher clock speeds and very competitive pricing to really save Phenom. You must log in or register to reply here. Currently have a low profile gtx with phenom ii x3 b CPUs 9 Nov 19, Post thread. Question Super giving me worse frames than a ? Started by Small-Change Oct 7, Replies: Last edited by Terwin; at PM. Very good explanation of Errata is here! Must read whole page before that conclusion to get it right.

RiG1: Ryzen 7 4. Must read whole page before that conclusion to get it right BTW great review! Last edited by justapost; at PM. Originally Posted by justapost.

Only read the conclusion so far. I wonder what hardware virtualisation functionality must be used. Xen 3. I could not trigger an error with win2k3 guests running prime Regardless, the two scenarios listed above aren't that rare and there could be more that trigger the problem, which makes a great case for fixing the problem.

The "TLB Bug" Explained Phenom is a monolithic quad core design, each of the four cores has its own internal L2 cache and the die has a single L3 cache that all of the cores share. If the CPU didn't cache page table entries, each memory access would proceed as follows: 1 Read from a pagetable directory 2 Read a pagetable entry 3 Then read the translated address and access memory Then there's something called a Translation Lookaside Buffer TLB which takes the addresses and maps them one to one, so you don't even need to perform a cache lookup - there's just a direct translation stored in the TLB.

Post Your Comment Please log in or sign up to comment. They say they got their hands on a 2. The CPUZ data confirms that. But in the final "extreme" test, they show the B3 2. So, they overclocked?? Via unlocked multiplier or by increasing the FSB???

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