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Date:   Mon, 14 Jan 2019 11:30:16 +0000
From:   Hedi Berriche <hedi.berriche@....com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Russ Anderson <rja@....com>, Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <sivanich@....com>,
        Steve Wahl <steve.wahl@....com>,
        Bhupesh SHARMA <bhupesh.linux@...il.com>
Subject: Re: [PATCH 3/3] x86/platform/UV: use efi_runtime_sem to serialise
 BIOS calls

On Thu, Jan 10, 2019 at 07:14 Bhupesh Sharma wrote:
>Hi Hedi,

Hi Bhupesh,

>Thanks for the patchset.

Thanks for looking at it.

>I will give this a go on my sgi-uv300 machine and come back with more 
>detailed inputs,

and for testing it.

>but I wanted to ask about the hang/panic you 
>mentioned in the cover letter when efi_scratch gets clobbered. Can you 
>describe the same (for e.g. how to reproduce this).

When efi_switch_mm() gets called concurrently from two different CPUs
--via arch_efi_call_virt_setup()-- due to lack of serialisation in
uv_bios_call(), efi_scratch.prev_mm is overwritten and that's how all
hell breaks loose, and that's when you see either a hang (the more
frequent failure mode) or a panic.

In order to reproduce the problem you'd need, for example, a kernel
module that makes use of uv_bios_call(), in which case a test case
would be a loop with:

	- 2 concurrent tasks both invoking uv_bios_call()

or
	- 2 concurrent tasks
		- one invoking uv_bios_call()
		- one, for example, accessing an EFI vars via efivars

>Nitpicks below:
>
>
>On 01/09/2019 04:15 PM, Hedi Berriche wrote:
>>Calls into UV firmware must be protected against concurrency, use the
>>now visible efi_runtime_sem lock to serialise them.
>>
>>Signed-off-by: Hedi Berriche <hedi.berriche@....com>
>>Reviewed-by: Russ Anderson <rja@....com>
>>Reviewed-by: Mike Travis <mike.travis@....com>
>>Reviewed-by: Dimitri Sivanich <sivanich@....com>
>>Reviewed-by: Steve Wahl <steve.wahl@....com>
>>---
>>  arch/x86/include/asm/uv/bios.h |  3 ++-
>>  arch/x86/platform/uv/bios_uv.c | 25 ++++++++++++++++++++++---
>>  2 files changed, 24 insertions(+), 4 deletions(-)
>>
>>diff --git a/arch/x86/include/asm/uv/bios.h b/arch/x86/include/asm/uv/bios.h
>>index 4eee646544b2..33e94aa0b1ff 100644
>>--- a/arch/x86/include/asm/uv/bios.h
>>+++ b/arch/x86/include/asm/uv/bios.h
>>@@ -48,7 +48,8 @@ enum {
>>  	BIOS_STATUS_SUCCESS		=  0,
>>  	BIOS_STATUS_UNIMPLEMENTED	= -ENOSYS,
>>  	BIOS_STATUS_EINVAL		= -EINVAL,
>>-	BIOS_STATUS_UNAVAIL		= -EBUSY
>>+	BIOS_STATUS_UNAVAIL		= -EBUSY,
>>+	BIOS_STATUS_ABORT		= -EINTR
>>  };
>>  /* Address map parameters */
>>diff --git a/arch/x86/platform/uv/bios_uv.c b/arch/x86/platform/uv/bios_uv.c
>>index cd05af157763..92f960798e20 100644
>>--- a/arch/x86/platform/uv/bios_uv.c
>>+++ b/arch/x86/platform/uv/bios_uv.c
>>@@ -29,7 +29,8 @@
>>  struct uv_systab *uv_systab;
>>-s64 uv_bios_call(enum uv_bios_cmd which, u64 a1, u64 a2, u64 a3, u64 a4, u64 a5)
>>+s64 __uv_bios_call(enum uv_bios_cmd which, u64 a1, u64 a2, u64 a3,
>>+			u64 a4, u64 a5)
>
>Can we make this static?

Will do.

>>  {
>>  	struct uv_systab *tab = uv_systab;
>>  	s64 ret;
>>@@ -44,13 +45,26 @@ s64 uv_bios_call(enum uv_bios_cmd which, u64 a1, u64 a2, u64 a3, u64 a4, u64 a5)
>>  	 * If EFI_OLD_MEMMAP is set, we need to fall back to using our old EFI
>>  	 * callback method, which uses efi_call() directly, with the kernel page tables:
>>  	 */
>>-	if (unlikely(test_bit(EFI_OLD_MEMMAP, &efi.flags)))
>>+	if (unlikely(efi_enabled(EFI_OLD_MEMMAP)))
>>  		ret = efi_call((void *)__va(tab->function), (u64)which, a1, a2, a3, a4, a5);
>>  	else
>>  		ret = efi_call_virt_pointer(tab, function, (u64)which, a1, a2, a3, a4, a5);
>>  	return ret;
>>  }
>>+
>>+s64 uv_bios_call(enum uv_bios_cmd which, u64 a1, u64 a2, u64 a3, u64 a4, u64 a5)
>>+{
>>+	s64 ret;
>>+
>>+	if (down_interruptible(&efi_runtime_sem))
>>+		return BIOS_STATUS_ABORT;
>>+
>>+	ret = __uv_bios_call(which, a1, a2, a3, a4, a5);
>>+	up(&efi_runtime_sem);
>>+
>>+	return ret;
>>+}
>>  EXPORT_SYMBOL_GPL(uv_bios_call);
>>  s64 uv_bios_call_irqsave(enum uv_bios_cmd which, u64 a1, u64 a2, u64 a3,
>>@@ -59,10 +73,15 @@ s64 uv_bios_call_irqsave(enum uv_bios_cmd which, u64 a1, u64 a2, u64 a3,
>>  	unsigned long bios_flags;
>>  	s64 ret;
>>+	if (down_interruptible(&efi_runtime_sem))
>>+		return BIOS_STATUS_ABORT;
>>+
>>  	local_irq_save(bios_flags);
>>-	ret = uv_bios_call(which, a1, a2, a3, a4, a5);
>>+	ret = __uv_bios_call(which, a1, a2, a3, a4, a5);
>>  	local_irq_restore(bios_flags);
>>+	up(&efi_runtime_sem);
>>+
>>  	return ret;
>>  }
>
>Thanks,
>Bhupesh

Cheers,
Hedi.
-- 
Be careful of reading health books, you might die of a misprint.
	-- Mark Twain

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