[PATCH v3 5/5] arm: qemu: override flash accessors to use virtualizable instructions

[André Przywara]

On 07/07/2020 11:07, Ard Biesheuvel wrote:
> Some instructions in the ARM ISA have multiple output registers, such
> as ldrd/ldp (load pair), where two registers are loaded from memory,
> but also ldr with indexing, where the memory base register is incremented
> as well when the value is loaded to the destination register.
> 
> MMIO emulation under KVM is based on using the architecturally defined
> syndrome information that is provided when an exception is taken to the
> hypervisor. This syndrome information describes whether the instruction
> that triggered the exception is a load or a store, what the faulting
> address was, and which register was the destination register.
> 
> This syndrome information can only describe one destination register, and
> when the trapping instruction is one with multiple outputs, KVM throws an
> error like
> 
>   kvm [615929]: Data abort outside memslots with no valid syndrome info
> 
> on the host and kills the QEMU process with the following error:
> 
>   U-Boot 2020.07-rc3-00208-g88bd5b179360-dirty (Jun 06 2020 - 11:59:22 +0200)
> 
>   DRAM:  1 GiB
>   Flash: error: kvm run failed Function not implemented
>   R00=00000001 R01=00000040 R02=7ee0ce20 R03=00000000
>   R04=7ffd9eec R05=00000004 R06=7ffda3f8 R07=00000055
>   R08=7ffd9eec R09=7ef0ded0 R10=7ee0ce20 R11=00000000
>   R12=00000004 R13=7ee0cdf8 R14=00000000 R15=7ff72d08
>   PSR=200001d3 --C- A svc32
>   QEMU: Terminated
> 
> This means that, in order to run U-Boot in QEMU under KVM, we need to
> avoid such instructions when accessing emulated devices. For the flash
> in particular, which is a hybrid between a ROM (backed by a read-only
> KVM memslot) when in array mode, and an emulated MMIO device (when in
> write mode), we need to take care to only use instructions that KVM can
> deal with when they trap.
> 
> So override the flash read accessors that are used when running on QEMU
> under KVM. Note that the the 64-bit wide read and write accessors have
> been omitted: they are never used when running under QEMU given that it
> does not emulate CFI flash that supports it.

Yes, that issue causes quite some headaches. In the Linux kernel the
MMIO accessors were deliberately chosen to be the ldr/str instructions
(as in this patch), to avoid this issue in the first place. So MMIO from
a Linux guest always works.
So it only makes sense to follow suit there. On the next occasion we
could actually think about moving the standard accessors over as well,
but for now this is good enough.

> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

Reviewed-by: Andre Przywara <andre.przywara at arm.com>

Cheers,
Andre

> ---
>  board/emulation/qemu-arm/qemu-arm.c | 45 ++++++++++++++++++++
>  include/configs/qemu-arm.h          |  1 +
>  2 files changed, 46 insertions(+)
> 
> diff --git a/board/emulation/qemu-arm/qemu-arm.c b/board/emulation/qemu-arm/qemu-arm.c
> index 1b0d543b93c1..f18f2ed7da3a 100644
> --- a/board/emulation/qemu-arm/qemu-arm.c
> +++ b/board/emulation/qemu-arm/qemu-arm.c
> @@ -142,3 +142,48 @@ efi_status_t platform_get_rng_device(struct udevice **dev)
>  	return EFI_SUCCESS;
>  }
>  #endif /* CONFIG_EFI_RNG_PROTOCOL */
> +
> +#ifdef CONFIG_ARM64
> +#define __W	"w"
> +#else
> +#define __W
> +#endif
> +
> +u8 flash_read8(void *addr)
> +{
> +	u8 ret;
> +
> +	asm("ldrb %" __W "0, %1" : "=r"(ret) : "m"(*(u8 *)addr));
> +	return ret;
> +}
> +
> +u16 flash_read16(void *addr)
> +{
> +	u16 ret;
> +
> +	asm("ldrh %" __W "0, %1" : "=r"(ret) : "m"(*(u16 *)addr));
> +	return ret;
> +}
> +
> +u32 flash_read32(void *addr)
> +{
> +	u32 ret;
> +
> +	asm("ldr %" __W "0, %1" : "=r"(ret) : "m"(*(u32 *)addr));
> +	return ret;
> +}
> +
> +void flash_write8(u8 value, void *addr)
> +{
> +	asm("strb %" __W "1, %0" : "=m"(*(u8 *)addr) : "r"(value));
> +}
> +
> +void flash_write16(u16 value, void *addr)
> +{
> +	asm("strh %" __W "1, %0" : "=m"(*(u16 *)addr) : "r"(value));
> +}
> +
> +void flash_write32(u32 value, void *addr)
> +{
> +	asm("str %" __W "1, %0" : "=m"(*(u32 *)addr) : "r"(value));
> +}
> diff --git a/include/configs/qemu-arm.h b/include/configs/qemu-arm.h
> index 1ef75a87836b..bc8b7c5c1238 100644
> --- a/include/configs/qemu-arm.h
> +++ b/include/configs/qemu-arm.h
> @@ -53,5 +53,6 @@
>  #define CONFIG_SYS_MAX_FLASH_BANKS	2
>  #endif
>  #define CONFIG_SYS_MAX_FLASH_SECT	256 /* Sector: 256K, Bank: 64M */
> +#define CONFIG_CFI_FLASH_USE_WEAK_ACCESSORS
>  
>  #endif /* __CONFIG_H */
>