On 05.02.24 19:46, Sean Christopherson wrote: > On Sat, Feb 03, 2024, Mathias Krause wrote: >> The ioctl()s to get and set KVM's debug registers are broken for 32 bit >> kernels as they'd only copy half of the user register state because of a >> UAPI and in-kernel type mismatch (__u64 vs. unsigned long; 8 vs. 4 >> bytes). >> >> This makes it impossible for userland to set anything but DR0 without >> resorting to bit folding tricks. >> >> Switch to a loop for copying debug registers that'll implicitly do the >> type conversion for us, if needed. >> >> There are likely no users (left) for 32bit KVM, fix the bug nonetheless. > > And this has always been broken, Jepp, that's why the fixes tag mentions the commit introducing the API. I also mentioned it already last year, tho[1]: "... The bug (existing since the introduction of the API) effectively makes using DR1..3 impossible." [1] https://lore.kernel.org/kvm/20230220104050.419438-1-minipli@xxxxxxxxxxxxxx/ > so if there were ever users of 32-bit KVM, they > obviously didn't use this API :-) Well, I do remember having issues with hardware breakpoints in combination with 32 bit guests. But that was *years* ago -- maybe even decades. Man, I'm old! > > If the code weren't also a cleanup for 64-bit, I would vote to change the APIs > to just fail for 32-bit. But there's just no good reason to assume that the > layouts of KVM's internal storage and "struct kvm_debugregs" are identical. Thanks!
