Roman Kagan <rkagan@xxxxxxxxxxxxx> writes: > On Fri, Nov 30, 2018 at 01:15:11PM +0100, Vitaly Kuznetsov wrote: >> Without 'packed' compiler is free to add optimization paddings and re-order >> structure fields for randomization/optimization. And structures from >> hyperv-tlfs.h are used for hypervisor-guest communication, we need to >> ultimately forbid such practices. > > Note that __packed also reduces the structure alignment to 1, which is > not necessarily what you want. > > E.g. some of these structures are passed by pointer to the hypercall, > which requires its arguments to be 8byte-aligned. Hm, I thought we always take precautions for Hyper-V hypercall arguments, in particular PV IPI/TLB flush use pre-allocated hyperv_pcpu_input_arg, hv_post_message() uses pre-allocated message page, other call sites use fast hypercalls where we use registers. I also checked this patch before sending out, WS2016 guest boots without issues. Any particular places you're worried about? > I'm also not sure > that passing unaligned argument to [rw]msr is ok, need to > double-check. My understanding is that rdmsr/wrmsr instuctions are registers-only. We can, of course, just add __aligned(8) to some structures but I'd like to find the reason first. -- Vitaly
