linux-kernel - Re: [PATCH v3 47/75] x86/sev-es: Add Runtime #VC Exception Handler

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Date:   Sat, 23 May 2020 09:59:24 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 47/75] x86/sev-es: Add Runtime #VC Exception Handler

On Tue, Apr 28, 2020 at 05:16:57PM +0200, Joerg Roedel wrote:
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index a4fa7f351bf2..bc3a58427028 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -10,6 +10,7 @@
>  #include <linux/sched/debug.h>	/* For show_regs() */
>  #include <linux/percpu-defs.h>
>  #include <linux/mem_encrypt.h>
> +#include <linux/lockdep.h>
>  #include <linux/printk.h>
>  #include <linux/mm_types.h>
>  #include <linux/set_memory.h>
> @@ -25,7 +26,7 @@
>  #include <asm/insn-eval.h>
>  #include <asm/fpu/internal.h>
>  #include <asm/processor.h>
> -#include <asm/trap_defs.h>
> +#include <asm/traps.h>
>  #include <asm/svm.h>
>  
>  /* For early boot hypervisor communication in SEV-ES enabled guests */
> @@ -46,10 +47,26 @@ struct sev_es_runtime_data {
>  
>  	/* Physical storage for the per-cpu IST stacks of the #VC handler */
>  	struct vmm_exception_stacks vc_stacks __aligned(PAGE_SIZE);
> +
> +	/* Reserve on page per CPU as backup storage for the unencrypted GHCB */

		  one

> +	struct ghcb backup_ghcb;

I could use some text explaining what those backups are for?

> +	/*
> +	 * Mark the per-cpu GHCBs as in-use to detect nested #VC exceptions.
> +	 * There is no need for it to be atomic, because nothing is written to
> +	 * the GHCB between the read and the write of ghcb_active. So it is safe
> +	 * to use it when a nested #VC exception happens before the write.
> +	 */

Looks liks that is that text... support for nested #VC exceptions.
I'm sure this has come up already but why do we even want to support
nested #VCs? IOW, can we do without them first or are they absolutely
necessary?

I'm guessing VC exceptions inside the VC handler but what are the
sensible use cases?

Thx.

-- 
Regards/Gruss,
    Boris.

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