Re: [PATCH 0/3] Use mm_struct and switch_mm() instead of manually

From: Bhupesh Sharma
Date: Sun Dec 17 2017 - 16:10:04 EST

Hi Sai,

On Sun, Dec 17, 2017 at 5:35 AM, Sai Praneeth Prakhya
<sai.praneeth.prakhya@xxxxxxxxx> wrote:
> From: Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx>
> Presently, in x86, to invoke any efi function like
> efi_set_virtual_address_map() or any efi_runtime_service() the code path
> typically involves read_cr3() (save previous pgd), write_cr3()
> (write efi_pgd) and calling efi function. Likewise after returning from
> efi function the code path typically involves read_cr3() (save efi_pgd),
> write_cr3() (write previous pgd). We do this couple of times in efi
> subsystem of Linux kernel, instead we can use helper function
> efi_switch_mm() to do this. This improves readability and maintainability.
> Also, instead of maintaining a separate struct "efi_scratch" to store/restore
> efi_pgd, we can use mm_struct to do this.
> I have tested this patch set against LUV (Linux UEFI Validation), so I
> think I didn't break any existing configurations. I have tested this
> patch set for
> 1. x86_64,
> 2. x86_32,
> 3. Mixed mode
> with efi=old_map and for kexec kernel. Please let me know if I have
> missed any other configurations.
> Changes in V2:
> 1. Resolve mm_dropping() issue by not mm_dropping()/mm_grabbing() any mm,
> as we are not losing/creating any references.
> Changes in V3:
> 1. When CPUMASK_OFFSTACK is enabled, switch_mm_irqs_off() sets cpumask
> by calling cpumask_set_cpu(). This panics kernel as efi_mm is not
> initialized, therefore initialize efi_mm in efi_alloc_page_tables().

Thanks for the v3.

I confirmed that the issue I saw with the v2 when I enabled
'efi=debug' on the sgi-uv 300 machine (i.e the NULL pointer access
while accessing mm_cpumask(next), in the function call
'switch_mm_irqs_off') is fixed in the v3.

Also as I noted during the v2 review, introducing the 'efi_switch_mm()
' helper instead of manually
twiddling with %cr3 seems more cleaner (having personally debugged
this leg several times on the underlying x86 EFI machines).

So in addition to me testing this on the sgi-uv300 and Dell Optiplex
EFI enabled machine, please feel free to add:

Reviewed-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>


> Note:
> This patch set is based on Linus's tree v4.15-rc3
> Sai Praneeth (3):
> efi: Use efi_mm in x86 as well as ARM
> x86/efi: Replace efi_pgd with efi_mm.pgd
> x86/efi: Use efi_switch_mm() rather than manually twiddling with %cr3
> arch/x86/include/asm/efi.h | 29 +++++++++---------
> arch/x86/platform/efi/efi_64.c | 59 +++++++++++++++++++-----------------
> arch/x86/platform/efi/efi_thunk_64.S | 2 +-
> drivers/firmware/efi/arm-runtime.c | 9 ------
> drivers/firmware/efi/efi.c | 9 ++++++
> include/linux/efi.h | 2 ++
> 6 files changed, 57 insertions(+), 53 deletions(-)
> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
> Cc: Lee, Chun-Yi <jlee@xxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Ricardo Neri <ricardo.neri@xxxxxxxxx>
> Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Ravi Shankar <ravi.v.shankar@xxxxxxxxx>
> Tested-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> --
> 2.1.4
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