RE: [boot-wrapper PATCH 5/5] aarch64: Introduce EL2 boot code for Armv8-R AArch64

quoted

Hi Andre,

-----Original Message-----
From: Andre Przywara <andre.przywara@arm.com>
Sent: Tuesday, May 11, 2021 4:00 PM
To: Jaxson Han <redacted>
Cc: Mark Rutland <Mark.Rutland@arm.com>; linux-arm-
kernel@lists.infradead.org; Wei Chen [off-list ref]
Subject: Re: [boot-wrapper PATCH 5/5] aarch64: Introduce EL2 boot code for
Armv8-R AArch64

On Tue, 11 May 2021 02:03:32 +0000
Jaxson Han [off-list ref] wrote:

Hi,

-----Original Message-----
From: Andre Przywara <andre.przywara@arm.com>
Sent: Monday, May 10, 2021 4:55 PM
To: Jaxson Han <redacted>
Cc: Mark Rutland <Mark.Rutland@arm.com>; linux-arm-
kernel@lists.infradead.org; Wei Chen [off-list ref]
Subject: Re: [boot-wrapper PATCH 5/5] aarch64: Introduce EL2 boot
code for Armv8-R AArch64

On Mon, 10 May 2021 02:13:45 +0000
Jaxson Han [off-list ref] wrote:

Hi Andre,

Since GCC 11 has been released and GCC 11 supports the '
-march=armv8-r', we got a problem when compile the boot-wrapper
with '
-march=armv8-r':
| ../git/arch/aarch64/boot.S:71: Error: selected processor does
| not support
system register name 'scr_el3'
| ../git/arch/aarch64/boot.S:73: Error: selected processor does
| not support
system register name 'cptr_el3'
| ../git/arch/aarch64/boot.S:84: Error: selected processor does
| not support
system register name 'mdcr_el3'
| ../git/arch/aarch64/boot.S:90: Error: selected processor does
| not support
system register name 'cptr_el3'
| ../git/arch/aarch64/boot.S:92: Error: selected processor does
| not support
system register name 'cptr_el3'
| ../git/arch/aarch64/boot.S:194: Error: selected processor does
| not
support system register name 'elr_el3'
| ../git/arch/aarch64/boot.S:195: Error: selected processor does
| not
support system register name 'spsr_el3'
It seems we may need some #if macro to disable all _el3 registers,
but it will break our auto-detection (users should add more
compile/build
parameter).
So, may I ask your suggestions? :)
Why do you need that in the first place? I think your version worked
without it? At least Ubuntu's 9.3.0 compiled it just fine.
Or does GCC 11 complain about some v8-r specific registers and you
need to add this armv8-r to let them pass, sacrificing all EL3 registers on
the way?
The problem comes from AIS yocto-bsp team. The reason they need this,
I think, may be that they want to test this new option for v8-r since
GCC 11 supports v8-r. Anyway, the current version works well. And I
agree it's neither necessary nor first priority to solve this problem
for boot-wrapper. But I think it's worth to discuss with you and see
your suggestions :)
Well, but each software package sets the stage for the compiler options it can
or cannot accept. And since the boot-wrapper is still foremost a v8-A
software, just compiling it with v8-r (or any other random switch) won't work.
So it's just not valid to use that switch there - it's the task of the Makefile to
set compiler options, any user choices have no guarantee of working anyway.
I got it. And we decided remove this option, because it's almost no gain.

And out of curiosity: what did you expect from using that option?
At the very beginning, maybe, expecting reorganization of v8-r registers,
or some optimizations?
But, since boot-wrapper does nothing but some inits, this option seems
useless totally.
So, we remove it.


One solution could be to move all accesses to v8-r registers into a
separate file, and only assemble/compile this with the v8-r switch.
But this sounds like some serious plumbing in the code base.

What you could try as well is to use this "s3_0_c12_c12_5" like
system register encoding style (this example is for ICC_SRE_EL1).
The kernel uses this trick to avoid dependencies on gas knowing
about all (new) system register names. Not sure if that is enough to trick
gas into accepting it?
Hope that helps.
Yes, it helps! Based on this, we could easily evaluate the efforts:)
Did you change the EL3 registers this way? Was there any effect on the code?
No, I didn't. There's no need to try, I think, with so many efforts and no gain. :)

Thanks,
Andre

Cheers,
Andre

Cheers,
Andre

Cheers,
Jaxson

-----Original Message-----
From: Jaxson Han
Sent: Wednesday, April 28, 2021 11:44 AM
To: Andre Przywara <andre.przywara@arm.com>
Cc: Mark Rutland <Mark.Rutland@arm.com>; linux-arm-
kernel@lists.infradead.org; Wei Chen [off-list ref]
Subject: RE: [boot-wrapper PATCH 5/5] aarch64: Introduce EL2
boot code for Armv8-R AArch64

Hi Andre,

-----Original Message-----
From: Andre Przywara <andre.przywara@arm.com>
Sent: Monday, April 26, 2021 8:36 PM
To: Jaxson Han <redacted>
Cc: Mark Rutland <Mark.Rutland@arm.com>; linux-arm-
kernel@lists.infradead.org; Wei Chen [off-list ref]
Subject: Re: [boot-wrapper PATCH 5/5] aarch64: Introduce EL2
boot code for Armv8-R AArch64

On Tue, 20 Apr 2021 15:24:38 +0800 Jaxson Han
[off-list ref]
wrote:

Hi,

The Armv8-R AArch64 profile does not support the EL3 exception
level.
The Armv8-R AArch64 profile allows for an (optional)
VMSAv8-64 MMU at EL1, which allows to run off-the-shelf
Linux. However EL2 only supports a PMSA, which is not
supported by Linux, so we need to drop into EL1 before entering
the kernel.
The boot sequence is:
If CurrentEL == EL3, then goto EL3 initialisation and drop to lower
EL
  before entering the kernel.
If CurrentEL == EL2 && id_aa64mmfr0_el1.MSA == 0xf (Armv8-R
aarch64),
  then goto Armv8-R AArch64 initialisation and drop to EL1 before
  entering the kernel.
Else, no initialisation and keep the current EL before entering the
  kernel.

Signed-off-by: Jaxson Han <redacted>
---
 arch/aarch64/boot.S | 51
+++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/arch/aarch64/boot.S b/arch/aarch64/boot.S index
f7dbf3f..6961a2a 100644
--- a/arch/aarch64/boot.S
+++ b/arch/aarch64/boot.S
@@ -25,16 +25,22 @@ _start:
 	 * Boot sequence
 	 * If CurrentEL == EL3, then goto EL3 initialisation and drop to
 	 *   lower EL before entering the kernel.
+	 * If CurrentEL == EL2 && id_aa64mmfr0_el1.MSA == 0xf,
then goto
+	 *   Armv8-R AArch64 initialisation and drop to EL1 before
+	 *   entering the kernel.
 	 * Else, no initialisation and keep the current EL before
 	 *   entering the kernel.
 	 */
 	mrs	x0, CurrentEL
 	cmp	x0, #CURRENTEL_EL3
 	beq	el3_init
+	cmp	x0, #CURRENTEL_EL2
+	beq	el2_init
nitpick: I tend to compare against EL2, then use b.gt for EL3,
b.lt for
EL1 and b.eq for EL2 code. Saves you an extra cmp here.
Exactly, I will. Thanks!


 	/*
 	 * We stay in the current EL for entering the kernel
 	 */
+keep_el:
 	mov	w0, #1
 	ldr	x1, =flag_keep_el
 	str	w0, [x1]
@@ -112,6 +118,43 @@ el3_init:
 	str	w0, [x1]
 	b	el_max_init

+	/*
+	 * EL2 Armv8-R AArch64 initialisation
+	 */
+el2_init:
+	/* Detect Armv8-R AArch64 */
+	mrs	x1, id_aa64mmfr0_el1
+	ubfx	x1, x1, #48, #4			// MSA
+	/* 0xf means Armv8-R AArch64 */
+	cmp	x1, 0xf
+	bne	keep_el
Don't we need to also check bits[55:52], to have at least 0b0010?
IIUC the support for VMSA in EL1&0 is optional, and should be
checked before we proceed? VTCR_EL2[31] can only be set in the
0b0010 case.
Yes, it should be checked, I will add it.


+
+	mrs	x0, midr_el1
+	msr	vpidr_el2, x0
+
+	mrs	x0, mpidr_el1
+	msr	vmpidr_el2, x0
+
+	mov	x0, #(1 << 31)			// VTCR_MSA:
VMSAv8-64
support
+	msr	vtcr_el2, x0
+
+	/* Enable pointer authentication if present */
+	mrs	x1, id_aa64isar1_el1
+	ldr	x2, =(((0xff) << 24) | (0xff << 4))
Each feature only holds four bits, so the mask you shift should be
0xf.
Yes, I will fix.


+	and	x1, x1, x2
+	cbz	x1, 1f
+
+	mrs	x0, hcr_el2
Shouldn't we force HCR_EL2, instead of modifying it? Just to
make sure nothing unexpected traps into EL2, which we don't
handle very
well?
So basically just set bit 31 (RES1), plus those two bits on
top, if needed. But I also wonder about FIEN[47] and EnSCXT[53] ...
Right, we should force to set HCR_EL2. The API and APK is needed.
And I will also check if we need the FIEN[47] and EnSCXT[53].

Thanks,
Jaxson


Rest looks alright.

Cheers,
Andre

+	orr	x0, x0, #(1 << 40)		// AP key enable
+	orr	x0, x0, #(1 << 41)		// AP insn enable
+	msr	hcr_el2, x0
+
+1:	isb
+	mov	w0, #SPSR_KERNEL_EL1
+	ldr	x1, =spsr_to_elx
+	str	w0, [x1]
+	b	el_max_init
+
 el_max_init:
 	ldr	x0, =CNTFRQ
 	msr	cntfrq_el0, x0
@@ -169,10 +212,18 @@ jump_kernel:
 	 */
 	bfi	x4, x19, #5, #1

+	mrs	x5, CurrentEL
+	cmp	x5, #CURRENTEL_EL2
+	b.eq	1f
+
 	msr	elr_el3, x19
 	msr	spsr_el3, x4
 	eret

+1:	msr	elr_el2, x19
+	msr	spsr_el2, x4
+	eret
+
 	.ltorg

 	.data

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