From: Andrew Cooper <Andrew.Cooper3@citrix.com>
To: Oleksii <oleksii.kurochko@gmail.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
Gianluca Guida <gianluca@rivosinc.com>,
Bob Eshleman <bobbyeshleman@gmail.com>,
Alistair Francis <alistair.francis@wdc.com>,
Connor Davis <connojdavis@gmail.com>
Subject: Re: [PATCH v1 07/14] xen/riscv: introduce exception handlers implementation
Date: Mon, 23 Jan 2023 20:09:12 +0000 [thread overview]
Message-ID: <29b9b149-d3b3-922b-c17f-d86b7f949fca@citrix.com> (raw)
In-Reply-To: <ea3c256c0f5a7f09a2504c548e649a0cf0edcb43.camel@gmail.com>
On 23/01/2023 3:17 pm, Oleksii wrote:
> On Mon, 2023-01-23 at 11:50 +0000, Andrew Cooper wrote:
>> On 20/01/2023 2:59 pm, Oleksii Kurochko wrote:
>>> + /* Save context to stack */
>>> + REG_S sp, (RISCV_CPU_USER_REGS_OFFSET(sp) -
>>> RISCV_CPU_USER_REGS_SIZE) (sp)
>>> + addi sp, sp, -RISCV_CPU_USER_REGS_SIZE
>>> + REG_S t0, RISCV_CPU_USER_REGS_OFFSET(t0)(sp)
>> Exceptions on RISC-V don't adjust the stack pointer. This logic
>> depends
>> on interrupting Xen code, and Xen not having suffered a stack
>> overflow
>> (and actually, that the space on the stack for all registers also
>> doesn't overflow).
>>
>> Which might be fine for now, but I think it warrants a comment
>> somewhere
>> (probably at handle_exception itself) stating the expectations while
>> it's still a work in progress. So in this case something like:
>>
>> /* Work-in-progress: Depends on interrupting Xen, and the stack
>> being
>> good. */
>>
>>
>> But, do we want to allocate stemp right away (even with an empty
>> struct), and get tp set up properly?
>>
> I am not sure that I get you here about stemp. Could you please clarify
> a little bit.
Sorry - sscratch, not stemp - I got the name wrong.
All registers are the interrupted context, not Xen's context. This
includes the stack pointer, global pointer, and thread pointer.
Trap setup is supposed to stash Xen's tp in sscratch so on an
interrupt/exception, it can exchange sscratch with tp and recover the
stack pointer.
Linux plays games with having sscratch be 0 while in kernel and uses
this to determine whether the exception occurred in kernel or user
mode. This is massive can of re-entrancy bugs that appears to be baked
into the architecture.
I genuinely can't figure out a safe way to cope with a stack overflow,
or a bad tp, because it is not safe to a pagefault until the exception
prologue has completed. If you do, you'll switch back to the
interrupted task's tp and use that as if it were Xen's.
~Andrew
next prev parent reply other threads:[~2023-01-23 20:09 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-20 14:59 [PATCH v1 00/14] RISCV basic exception handling implementation Oleksii Kurochko
2023-01-20 14:59 ` [PATCH v1 01/14] xen/riscv: add _zicsr to CFLAGS Oleksii Kurochko
2023-01-20 15:29 ` Andrew Cooper
2023-01-23 10:43 ` Oleksii
2023-01-31 11:49 ` Alistair Francis
2023-01-31 12:30 ` Oleksii
2023-01-20 14:59 ` [PATCH v1 02/14] xen/riscv: add <asm/asm.h> header Oleksii Kurochko
2023-01-20 15:31 ` Andrew Cooper
2023-01-23 11:00 ` Jan Beulich
2023-01-23 11:10 ` Andrew Cooper
2023-01-22 22:58 ` Alistair Francis
2023-01-20 14:59 ` [PATCH v1 03/14] xen/riscv: add <asm/riscv_encoding.h header Oleksii Kurochko
2023-01-22 23:24 ` Alistair Francis
2023-01-23 13:52 ` Jan Beulich
2023-01-23 14:04 ` Oleksii
2023-01-23 14:06 ` Jan Beulich
2023-01-20 14:59 ` [PATCH v1 04/14] xen/riscv: add <asm/csr.h> header Oleksii Kurochko
2023-01-22 23:25 ` Alistair Francis
2023-01-23 13:57 ` Jan Beulich
2023-01-23 14:23 ` Oleksii
2023-01-23 14:31 ` Jan Beulich
2023-01-20 14:59 ` [PATCH v1 05/14] xen/riscv: add early_printk_hnum() function Oleksii Kurochko
2023-01-20 15:39 ` Andrew Cooper
2023-01-23 12:05 ` Oleksii
2023-01-23 11:10 ` Jan Beulich
2023-01-20 14:59 ` [PATCH v1 06/14] xen/riscv: introduce exception context Oleksii Kurochko
2023-01-20 15:54 ` Andrew Cooper
2023-01-23 12:03 ` Oleksii
2023-01-23 12:25 ` Andrew Cooper
2023-01-23 11:13 ` Jan Beulich
2023-01-20 14:59 ` [PATCH v1 07/14] xen/riscv: introduce exception handlers implementation Oleksii Kurochko
2023-01-22 23:29 ` Alistair Francis
2023-01-23 11:17 ` Jan Beulich
2023-01-23 15:04 ` Oleksii
2023-01-23 11:50 ` Andrew Cooper
2023-01-23 12:41 ` Jan Beulich
2023-01-23 15:17 ` Oleksii
2023-01-23 20:09 ` Andrew Cooper [this message]
2023-01-25 14:44 ` Oleksii
2023-01-20 14:59 ` [PATCH v1 08/14] xen/riscv: introduce decode_cause() stuff Oleksii Kurochko
2023-01-22 23:38 ` Alistair Francis
2023-01-23 12:09 ` Andrew Cooper
2023-01-20 14:59 ` [PATCH v1 09/14] xen/riscv: introduce do_unexpected_trap() Oleksii Kurochko
2023-01-22 23:39 ` Alistair Francis
2023-01-25 17:01 ` Oleksii
2023-01-25 17:11 ` Julien Grall
2023-01-25 17:15 ` Andrew Cooper
2023-01-26 8:40 ` Oleksii
2023-01-20 14:59 ` [PATCH v1 10/14] xen/riscv: mask all interrupts Oleksii Kurochko
2023-01-22 23:40 ` Alistair Francis
2023-01-20 14:59 ` [PATCH v1 11/14] xen/riscv: introduce setup_trap_handler() Oleksii Kurochko
2023-01-22 23:41 ` Alistair Francis
2023-01-23 23:21 ` Andrew Cooper
2023-01-20 14:59 ` [PATCH v1 12/14] xen/riscv: introduce an implementation of macros from <asm/bug.h> Oleksii Kurochko
2023-01-23 11:37 ` Jan Beulich
2023-01-20 14:59 ` [PATCH v1 13/14] xen/riscv: test basic handling stuff Oleksii Kurochko
2023-01-20 14:59 ` [PATCH v1 14/14] automation: add smoke test to verify macros from bug.h Oleksii Kurochko
2023-01-24 23:53 ` Stefano Stabellini
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