On Sat, Jun 4, 2016 at 12:00 PM, Zoran Stojsavljevic zoran.stojsavljevic@gmail.com wrote:
Hello to all,
If I correctly remember: PCIe configuration space addressing consists of 3 parts: bus (8 bits), device (5 bits) and function (3 bits). This gives in total 8+5+3= 16 bits, thus 2^16 (65536). With additional 256 bytes legacy, gives maximum of 16MB of configuration address space (just below TOM) for legacy. With 4KB per function (extended config space), gives 256MB (0x10000000). Here it is: https://en.wikipedia.org/wiki/PCI_configuration_space
For extended config space it's 256 MiB total.
The question about system memory beyond the PCIe bridge is the another question. Seems to me that 2GB is too much (not compliant with IA32/x86, only with x86_64 architecture). Thus I agree with Aaron. Hard to comply with IA32/x86.
But in the past I saw similar behavior with BIOSes (double reboot). Not sure if this is the real reason (Aaron writes: Intel UEFI systems do a reboot after calculating the MMIO space requirements so that the memory init code knows how much to allocate for MMIO resource allocation.). Hard to imagine for UEFI 32bit compliant BIOSes, all 64 BIOSes are UEFI compliant BIOSes.
If Aaron is correct, I've learned something new. ;-)
It's a trade off of accessible memory (DRAM) vs simplification. If your kernel you are booting is 64-bit it matters nothing at all using 2GiB I/O hole. If you care about 32-bit kernels then you care more about the size of the I/O hole. As i noted previously, I haven't been concerned with 32-bit kernels for over decade, but that's just what I'm used to and am concerned about. Obviously if 32-bit kernels are a concern then you wouldn't use a 2GiB hole when trying to maximize dram access.
Thank you, Zoran
On Sat, Jun 4, 2016 at 6:49 PM, ron minnich rminnich@gmail.com wrote:
Another Kconfig option? How many people will really understand what it means and whether to use it?
Has just reserving 2 GiB as a hard and fast rule hurt anyone yet?
thanks
ron
On Fri, Jun 3, 2016 at 11:25 PM Patrick Rudolph siro@das-labor.org wrote:
On 2016-06-03 05:41 PM, Aaron Durbin via coreboot wrote:
On Fri, Jun 3, 2016 at 7:04 AM, Patrick Rudolph siro@das-labor.org wrote:
Hello, I want to start a discussion about PCI MMIO size that hit me a couple of times using coreboot. I'm focused on Intel Sandybridge, but I guess this topic applies to all x86 systems.
On most Intel systems the PCI mmio size is hard-coded in coreboot to 1024Mbyte, but the value is hidden deep inside raminit code. The mmio size for dynamic resources is limited on top by PCIEXBAR, IOAPIC, ME stolen, ... that takes 128Mbyte and on the other end it's limited by graphics stolen and TSEG that require at least 40Mbyte. In total there's only space for two 256Mbyte PCI BARs, due to alignment. That's enough for systems that only do have an Intel GPU, but it fails as soon as PCI devices use more than a single 256Mbyte BAR. The PCI mmio size is set in romstage, but PCI BARs are configured in ramstage.
Following questions came to my mind:
- How does the MRC handle this ?
- Should the user be able to modify PCI mmio size ?
- How to pass the required PCI mmio size to romstage ? A good place seems to be the mrc cache, but ramstage doesn't know
about it's structure.
- How is this solved on AMD systems ?
- Should the romstage scan PCI devices and count required BAR size ?
In the past (not sure if it's still true), Intel UEFI systems do a reboot after calculating the MMIO space requirements so that the memory init code knows how much to allocate for MMIO resource allocation. I always found that distasteful and I have always used a 2GiB I/O hole ever since. I never cared about 32-bit kernels so I always found that to be a decent tradeoff. It also makes MTRR and address space easy to digest when looking at things.
I like the idea of a reboot. It only has to be done after hardware changes that affect the PCI mmio size. With mrc cache in place it shouldn't be notable at all.
On the other hand, hard-coding the limit is much simpler. What do you think about a Kconfig option "Optimize PCI mmio size for x86_64 OS" ?
It would increase the size to 2GiB. Of course it would work on i386, but you might see less usable DRAM than before.
Regards, Patrick
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