Re: [PATCH] x86/hyper-v: define structures from TLFS as packed

[Vitaly Kuznetsov]

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