And the PCIe device implicitly/inherently knows the address range size used by each BAR. And OS/FW/BIOS can know that by writing all 1s to the BAR register. With this base/size info, the PCIe device can accurately identify which BAR an arriving address falls into. And then properly calculate the offset from the arriving address.

Nov 22, 2022 · Which one is faster, DMA addressed by the PCIe device side or MMIO addressed by the host processor for both read and write (usually the DMA region is not that big since it needs contiguous host-side DRAM space for the same size as DMA region as the device does not understand host paging, but just for comparison), if we assume that all the ...

Jun 25, 2014 · There is more overlap between the PCIe spec and the uncore counters with regard to the "no snoop required" bit, but again the PCIe spec does not require any particular behavior -- a processor is allowed to snoop requests with the "no snoop required" bit set, and a processor is allowed to refrain from snooping requests with the "no snoop ...

Dec 28, 2020 · For cached MMIO (using the WT or WP memory types), the core can always perform a "flush after use" on cache line. For streaming loads from WC space, it may take some experimentation to find a way to ensure that stale data does not stay in the core's read buffer after the device has modified the target line.

Aug 12, 2024 · But when I try to use MMIO via UIO to access BAR space, the highest bandwidth is only 3.75 GBps. I have tried consecutive vmovntdqa, vmovapd, movdir64b, memcpy(), and I use ioremap_wc() in kernel driver to map a 4MB Write Combine space.

Each PCI device (when I write PCI, I refer to PCI 3.0, as opposed to PCIe) has two "ranges" - configuration range (CFG) and "memory mapped input-output" range (MMIO). I won't deep dive into the concepts of address spaces and MMIO because it will make the answer too long and complicated. Google them if they are not familiar to you.

Sep 3, 2015 · And how it's useful for PCIe functional operation ? This space contains BAR (base address register). Is this register gets used to specify the address available in PCIe endpoint ? I am new to the PCIe, and trying to learn it. I am referring the Base specification, But I think it's written for the readers having some prior knowledge of PCI and PCIe.

Aug 9, 2017 · Likely the OP has some unusual PCIe devices with large amounts of onboard high-speed memory that the BIOS thinks the CPU needs access to. The BIOS might be wrong about that, in which case a BIOS update might fix it. But the BIOS might also be right. I had this same issue with a desktop with a Xeon Phi coprocessor with lots of onboard RAM.

Jan 4, 2025 · As you can see, I use the pcie device on socket 1. There is correct PCIRd counter value and ItoM counter value. However, the counters of MMIO events(i.e. WiL) are extremely low(i.e. 0 and 20. How can I use the PCM tool to measure counters of MMIO event in this type of machines? Thank you in advance for your great help! Sincerely, Qiangsheng Su

Sep 13, 2016 · There is no way to absolutely guarantee a single 64-Byte packet, but if you use a Write-Combining memory type and issue a small number of consecutive writes (e.g., 2 32-Byte AVX/AVX2 stores) to 64 Bytes starting at a 64-Byte-aligned address, then you will get a single 64-Byte PCIe transaction *almost* all the time.