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SubjectRe: [PATCH v2 11/15] arm64: add EFI stub
On Wed, Mar 19, 2014 at 03:13:18PM +0000, Mark Salter wrote:
> On Wed, 2014-03-19 at 10:57 +0000, Catalin Marinas wrote:
> > On Tue, Mar 18, 2014 at 09:40:31PM +0000, Mark Salter wrote:
> > > On Tue, 2014-03-18 at 18:28 +0000, Catalin Marinas wrote:
> > > > If UEFI doesn't handle the caches, the only thing left to EFI_STUB is to
> > > > flush by MVA. We don't need to flush the whole DRAM (and I would even
> > > > recommend it) but at least the relevant kernel code/data touched with
> > > > the MMU disabled.
> > >
> > > So, it goes like this:
> > >
> > > 1) UEFI calls stub with MMU/Caches on. Stub/kernel can be anywhere.
> > > 2) Stub runs and relocates kernel to the desired runtime location
> > > but continues to execute from wherever UEFI loaded it until just
> > > after ExitBootServices().
> > > 3) After ExitBootServices, efi_entry() returns relocated entry point
> > > for kernel to efi_stub_entry() in efi-entry.S where the Dcache and
> > > MMU are turned off, the __flush_dcache_all is called, then the
> > > code jumps to the kernel proper entry point.
> > >
> > > It isn't clear to me if UEFI does cache flushing at ExitBootServices
> > > time, but even so, at least stack use will get cached between then and
> > > the kernel entry point. The stub could conceivably get its hands on the
> > > EFI memmap and invalidate dcache using address ranges from UEFI memory
> > > descriptors so maybe that is the way we should do it.
> >
> > I think the stub just needs to flush the relocated kernel image, ensure
> > it is sync with the memory. Additional flushing can be done by the
> > kernel for bits it writes (like page tables, code patching etc). We can
> > enter the kernel with the SCTLR.I bit set, so it can allocate in an
> > unified cache already and D-cache maintenance would be needed anyway.
>
> How about this?
>
> diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
> index 83bfb72..ed480b2 100644
> --- a/arch/arm64/kernel/efi-entry.S
> +++ b/arch/arm64/kernel/efi-entry.S
> @@ -52,11 +52,19 @@ ENTRY(efi_stub_entry)
> * efi_entry() will have relocated the kernel image if necessary
> * and we return here with device tree address in x0 and the kernel
> * entry point stored at *image_addr. Save those values in registers
> - * which are preserved by __flush_dcache_all.
> + * which are callee preserved.
> */
> - ldr x1, [sp, #16]
> mov x20, x0
> - mov x21, x1
> + ldr x0, [sp, #16]
> + mov x21, x0

BTW, some comments to make it easier to review the code later about
what's at [sp, #16].

> +
> + adrp x1, _text
> + add x1, x1, #:lo12:_text
> + adrp x2, _edata
> + add x2, x2, #:lo12:_edata
> + sub x1, x2, x1
> +
> + bl __flush_dcache_area

Looks fine. We also need a "ic ialluis", is any other part of EFI_STUB
invalidating the I-cache?

We need a patch to invalidate the D-cache for areas of memory that the
kernel touches but I'll do this as a separate patch.

--
Catalin


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