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Date:   Wed, 11 Jan 2023 12:20:14 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joan Bruguera <joanbrugueram@gmail.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Juergen Gross <jgross@suse.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>
Subject: Re: Wake-up from suspend to RAM broken under `retbleed=stuff`
Message-ID: <Y76bbtJn+jIV3pOz@hirez.programming.kicks-ass.net>
References: <20230108030748.158120-1-joanbrugueram@gmail.com>
 <20230109040531.7888-1-joanbrugueram@gmail.com>
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On Mon, Jan 09, 2023 at 04:05:31AM +0000, Joan Bruguera wrote:
> This fixes wakeup for me on both QEMU and real HW
> (just a proof of concept, don't merge)
> 
> diff --git a/arch/x86/kernel/callthunks.c b/arch/x86/kernel/callthunks.c
> index ffea98f9064b..8704bcc0ce32 100644
> --- a/arch/x86/kernel/callthunks.c
> +++ b/arch/x86/kernel/callthunks.c
> @@ -7,6 +7,7 @@
>  #include <linux/memory.h>
>  #include <linux/moduleloader.h>
>  #include <linux/static_call.h>
> +#include <linux/suspend.h>
>  
>  #include <asm/alternative.h>
>  #include <asm/asm-offsets.h>
> @@ -150,6 +151,10 @@ static bool skip_addr(void *dest)
>  	if (dest >= (void *)hypercall_page &&
>  	    dest < (void*)hypercall_page + PAGE_SIZE)
>  		return true;
> +#endif
> +#ifdef CONFIG_PM_SLEEP
> +	if (dest == restore_processor_state)
> +		return true;
>  #endif
>  	return false;
>  }
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index 236447ee9beb..e667894936f7 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -281,6 +281,9 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
>  /* Needed by apm.c */
>  void notrace restore_processor_state(void)
>  {
> +	/* Restore GS before calling anything to avoid crash on call depth accounting */
> +	native_wrmsrl(MSR_GS_BASE, saved_context.kernelmode_gs_base);
> +
>  	__restore_processor_state(&saved_context);
>  }

Yeah, I can see why, but I'm not really comfortable with this. TBH, I
don't see how the whole resume code is correct to begin with. At the
very least it needs a heavy dose of noinstr.

Rafael, what cr3 is active when we call restore_processor_state()?

Specifically, the problem is that I don't feel comfortable doing any
sort of weird code until all the CR and segment registers have been
restored, however, write_cr*() are paravirt functions that result in
CALL, which then gives us a bit of a checken and egg problem.

I'm also wondering how well retbleed=stuff works on Xen, if at all. If
we can ignore Xen, things are a little earier perhaps.