linux-kernel - Re: [PATCH v1 02/16] irqchip: gicv3-its: Add helpers for handling 52bit address

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Message-ID: <20180208134557.GQ29286@cbox>
Date:   Thu, 8 Feb 2018 14:45:57 +0100
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
        linux-kernel@...r.kernel.org, kristina.martsenko@....com,
        peter.maydell@...aro.org, pbonzini@...hat.com, rkrcmar@...hat.com,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        mark.rutland@....com, catalin.marinas@....com,
        Shanker Donthineni <shankerd@...eaurora.org>
Subject: Re: [PATCH v1 02/16] irqchip: gicv3-its: Add helpers for handling
 52bit address

On Thu, Feb 08, 2018 at 11:20:02AM +0000, Suzuki K Poulose wrote:
> On 07/02/18 15:10, Christoffer Dall wrote:
> >Hi Suzuki,
> >
> >On Tue, Jan 09, 2018 at 07:03:57PM +0000, Suzuki K Poulose wrote:
> >>Add helpers for encoding/decoding 52bit address in GICv3 ITS BASER
> >>register. When ITS uses 64K page size, the 52bits of physical address
> >>are encoded in BASER[47:12] as follows :
> >>
> >>  Bits[47:16] of the register => bits[47:16] of the physical address
> >>  Bits[15:12] of the register => bits[51:48] of the physical address
> >>                                 bits[15:0] of the physical address are 0.
> >>
> >>Also adds a mask for CBASER address. This will be used for adding 52bit
> >>support for VGIC ITS. More importantly ignore the upper bits if 52bit
> >>support is not enabled.
> >>
> >>Cc: Shanker Donthineni <shankerd@...eaurora.org>
> >>Cc: Marc Zyngier <marc.zyngier@....com>
> >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> >>---
> 
> 
> >>+
> >>+/*
> >>+ * With 64K page size, the physical address can be upto 52bit and
> >>+ * uses the following encoding in the GITS_BASER[47:12]:
> >>+ *
> >>+ * Bits[47:16] of the register => bits[47:16] of the base physical address.
> >>+ * Bits[15:12] of the register => bits[51:48] of the base physical address.
> >>+ *                                bits[15:0] of the base physical address are 0.
> >>+ * Clear the upper bits if the kernel doesn't support 52bits.
> >>+ */
> >>+#define GITS_BASER_ADDR64K_LO_MASK	GENMASK_ULL(47, 16)
> >>+#define GITS_BASER_ADDR64K_HI_SHIFT	12
> >>+#define GITS_BASER_ADDR64K_HI_MOVE	(48 - GITS_BASER_ADDR64K_HI_SHIFT)
> >>+#define GITS_BASER_ADDR64K_HI_MASK	(GITS_PA_HI_MASK << GITS_BASER_ADDR64K_HI_SHIFT)
> >>+#define GITS_BASER_ADDR64K_TO_PHYS(x)					\
> >>+	(((x) & GITS_BASER_ADDR64K_LO_MASK) | 				\
> >>+	 (((x) & GITS_BASER_ADDR64K_HI_MASK) << GITS_BASER_ADDR64K_HI_MOVE))
> >>+#define GITS_BASER_ADDR64K_FROM_PHYS(p)					\
> >>+	(((p) & GITS_BASER_ADDR64K_LO_MASK) | 				\
> >>+	 (((p) >> GITS_BASER_ADDR64K_HI_MOVE) & GITS_BASER_ADDR64K_HI_MASK))
> >
> >I don't understand why you need this masking logic embedded in these
> >macros?  Isn't it strictly an error if anyone passes a physical address
> >with any of bits [51:48] set to the ITS on a system that doesn't support
> >52 bit PAs, and just silently masking off those bits could lead to some
> >interesting cases.
> 
> What do you think is the best way to handle such cases ? May be I could add
> some checks where we get those addresses and handle it before we use this
> macro ?
> 

I don't think the conversion macros should try to hide programming
errors.  I think we should limit the functionality in the macros to be
simple bit masking and shifting.

Any validation and masking depending on 52 PA support in the kernel
should be done in the context of the functionality, just like the ITS
driver already does.

> >
> >This is also notably more difficult to read than the existing macro.
> >
> >If anything, I think it would be more useful to have
> >GITS_BASER_TO_PHYS(x) and GITS_PHYS_TO_BASER(x) which takes into account
> >CONFIG_ARM64_64K_PAGES.
> 
> I thought the 64K_PAGES is not kernel page size, but the page-size configured
> by the "requester" for ITS. So, it doesn't really mean CONFIG_ARM64_64K_PAGES.

You're right, I skimmed this logic too quickly.

> But the other way around, we can't handle 52bit address unless CONFIG_ARM64_64K_PAGES
> is selected. Also, if the guest uses a 4K page size and uses a 48 bit address,
> we could potentially mask Bits[15:12] to 0, which is not nice.
> 
> So I still think we need to have a special macro for handling addresses with 64K
> page size in ITS.
> 
I think it's easier to have the current GITS_BASER_PHYS_52_to_48 and
have a corresponding GITS_BASER_PHYS_48_to_52, following Robin's
observation.

Any additional logic can be written directly in the C code to check
consitency etc.

Thanks,
-Christoffer