get_bar_size(9)

📦 version 7.3.1, in nixpkgs unstable

💅 get_bar_size — Get the address space consumed by a BAR

okay so bestie, this page is get_bar_size(9) — that's the kernel API manual, and yes the manual name is literally get_bar_size too, so we're all caught up 💖. It lives in the kernel API docs and it's honestly kind of a tiny icon.

✨ SYNOPSIS ✨

u64 get_bar_size (u64 bar_val );

that's it. one function, one argument, one u64 coming back at you. obsessed 🎀

🫶 ARGUMENTS 🫶

  • bar_val — the value that a BAR handed back at you after -1 was written to it. that's the whole vibe check: write -1, see what it says, pass that in here 🦋

🌸 DESCRIPTION 🌸

so what this function does is it returns the size of the BAR, rounded up to 1 page. and ngl the rounding-up is not a stylistic choice, it's literally required — because the hypervisor's page table entry that maps the BAR into the VM can't specify an offset within a page. like, the hardware said no offsets, and we respect the hardware 💀

and here's the invariant you need to keep in your head, bestie, write it on your hand: the hypervisor must place any BARs of smaller than page length at the beginning of a page. that's the rule. periodt. 👑

⭐ RETURN ⭐

size in bytes of the consumed MMIO space. that's the answer, that's the number, that's the tea ☕

💜 SEE ALSO 💜

okay this list is long so buckle up, it's giving "the whole pci-hyperv.c friend group" honestly 😭

kernel file ./drivers/pci/controller/pci-hyperv.c, plus:

like I said. the whole friend group. no cap 🌈

fr fr bestie, if you're poking at BAR sizing in the hv pci frontend, you're in the right neighborhood — just remember the page-rounding rule and you're golden 💅✨

and that's the page! 🦋 this one came out of ./drivers/pci/controller and it's dated October 2026, with the page itself being get_bar_size(9) — mwah, go read the rest of the kernel API manual, ily, bye 🎀💖🌸

📦 where this page comes from