Re: [PATCH] target/loongarch: Support 4K page size

[Peter Maydell]

On Mon, 23 Oct 2023 at 05:06, maobibo <maobibo@loongson.cn> wrote:
>
>
>
> 在 2023/10/23 上午10:40, Song Gao 写道:
> > The LoongArch kernel supports 4K page size.
> > Change TARGET_PAGE_BITS to 12.
> >
> > Signed-off-by: Song Gao <gaosong@loongson.cn>
> > ---
> >   target/loongarch/cpu-param.h  | 2 +-
> >   target/loongarch/tlb_helper.c | 9 ++++-----
> >   2 files changed, 5 insertions(+), 6 deletions(-)
> >
> > diff --git a/target/loongarch/cpu-param.h b/target/loongarch/cpu-param.h
> > index 1265dc7cb5..cfe195db4e 100644
> > --- a/target/loongarch/cpu-param.h
> > +++ b/target/loongarch/cpu-param.h
> > @@ -12,6 +12,6 @@
> >   #define TARGET_PHYS_ADDR_SPACE_BITS 48
> >   #define TARGET_VIRT_ADDR_SPACE_BITS 48
> >
> > -#define TARGET_PAGE_BITS 14
> > +#define TARGET_PAGE_BITS 12
> Hi Gaosong,
>
> The popular OS about LoongArch still uses 16K page size, qemu should
> follow the rule of OS rather than defining 4K page size alone.

The TARGET_PAGE_BITS value in QEMU is a property of the hardware,
not the guest OS. It should specify the smallest page size the
guest can configure the CPU to use. If the guest asks for a
larger page size than the minimum then that works fine. See
for example PPC64 -- on this architecture both 4K and 64K
pages are possible, so we define TARGET_PAGE_BITS to 12,
even though a lot of Linux guests use 64K pages.

It is slightly less efficient when the guest uses a page size
larger than the TARGET_PAGE_BITS value indicates, so if you
have an architecture where some CPUs support small pages
but most do not, you can do what Arm does, and use the
TARGET_PAGE_BITS_VARY support. This makes the TARGET_PAGE_BITS
macro be a runtime-configurable value, where a machine model can
set the mc->minimum_page_bits value to indicate that that
machine doesn't need the small-pages handling.

thanks
-- PMM