Re: [PATCH v11 1/7] x86/kvm/hyper-v: Explicitly align hcall param for kvm_hyperv_exit

[Roman Kagan <rvkagan@yandex-team.ru>]

On Fri, Apr 24, 2020 at 02:37:40PM +0300, Jon Doron wrote:
> The problem the patch is trying to address is the fact that 'struct
> kvm_hyperv_exit' has different layout on when compiling in 32 and 64 bit
> modes.
> 
> In 64-bit mode the default alignment boundary is 64 bits thus
> forcing extra gaps after 'type' and 'msr' but in 32-bit mode the
> boundary is at 32 bits thus no extra gaps.
> 
> This is an issue as even when the kernel is 64 bit, the userspace using
> the interface can be both 32 and 64 bit but the same 32 bit userspace has
> to work with 32 bit kernel.
> 
> The issue is fixed by forcing the 64 bit layout, this leads to ABI
> change for 32 bit builds and while we are obviously breaking '32 bit
> userspace with 32 bit kernel' case, we're fixing the '32 bit userspace
> with 64 bit kernel' one.
> 
> As the interface has no (known) users and 32 bit KVM is rather baroque
> nowadays, this seems like a reasonable decision.
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Jon Doron <arilou@gmail.com>
> ---
>  Documentation/virt/kvm/api.rst | 2 ++
>  include/uapi/linux/kvm.h       | 2 ++
>  2 files changed, 4 insertions(+)

Reviewed-by: Roman Kagan <rvkagan@yandex-team.ru>