Re: [Qemu-devel] [PATCH] kvm-all: PAGE_SIZE should be real host page size

[Andrew Jones <drjones@redhat.com>]

On Tue, Nov 10, 2015 at 04:29:31PM +0000, Peter Maydell wrote:
> On 10 November 2015 at 00:23, Andrew Jones <drjones@redhat.com> wrote:
> > Just noticed this while grepping TARGET_PAGE_SIZE for an unrelated
> > reason. I didn't use qemu_real_host_page_size as kvm_set_phys_mem()
> > does, because we'd need to make sure page_size_init() has run first.
> >
> > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > ---
> >  kvm-all.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/kvm-all.c b/kvm-all.c
> > index 1bc12737723c3..de9ff5971fb3b 100644
> > --- a/kvm-all.c
> > +++ b/kvm-all.c
> > @@ -45,8 +45,10 @@
> >  #include <sys/eventfd.h>
> >  #endif
> >
> > -/* KVM uses PAGE_SIZE in its definition of COALESCED_MMIO_MAX */
> > -#define PAGE_SIZE TARGET_PAGE_SIZE
> > +/* KVM uses PAGE_SIZE in its definition of KVM_COALESCED_MMIO_MAX. We
> > + * need to use the real host PAGE_SIZE, as that's what KVM will use.
> > + */
> > +#define PAGE_SIZE getpagesize()
> 
> Rather than defining PAGE_SIZE here (a confusing macro given
> we have several page sizes to deal with), why not just use
> getpagesize() in the one and only location where we currently
> use this macro?

The macro is used by kernel headers that we import and include in
kvm-all.c. It's ugly, I agree, but that's how the this cookie crumbled.

> 
> Also, you're guaranteed that page_size_init() has been run, because
> we call that from kvm_init(), and you can't call kvm_vcpu_init()
> before kvm_init().

True, but having that dependency seemed error prone to me. If we
we some day changed when/if page_size_init is called then there
could be an issue, or if somebody did something like

kvm_init()
{
  my_page_size = PAGE_SIZE;
  ...
  page_size_init();
  ...
  use(my_page_size)
}

things would break.

Thanks,
drew