From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
To: Ian Campbell <ian.campbell@citrix.com>
Cc: Julien Grall <julien.grall@citrix.com>,
xen-devel@lists.xen.org, stefano.stabellini@eu.citrix.com
Subject: Re: [PATCH v2] xen/arm64: correctly emulate the {w, x}zr registers
Date: Mon, 22 Feb 2016 17:38:15 +0000 [thread overview]
Message-ID: <alpine.DEB.2.02.1602221655060.17248@kaball.uk.xensource.com> (raw)
In-Reply-To: <1452871034-9481-1-git-send-email-ian.campbell@citrix.com>
On Fri, 15 Jan 2016, Ian Campbell wrote:
> From: Julien Grall <julien.grall@citrix.com>
>
> On AArch64, encoding 31 for an R<n> in the HSR is used to represent
> either {w,x}sp or {w,x}zr (See C1.2.4 in ARM DDI 0486A.d) depending on
> how the register field is interpreted by the instruction.
>
> All the instructions trapped by Xen (either via a sysreg access or
> data abort) interpret encoding 31 as {w,x}zr. Therefore we don't have
> to worry about the possibility that a trap could refer to sp or about
> decoding the instruction.
>
> For example AArch64 LDR and STR can have zr in the source/target
> register <Xt>, but never sp. sp can be present in the destination
> pointer( i.e. "[sp]"), but that would be represented by the value of
> FAR_EL2, not in the HSR.
>
> For AArch32 it is possible for a LDR to target the PC, but this would
> not result in a valid ISS in the HSR register. However this could only
> occur if loading or storing the PC to MMIO, which we simply choose not
> to support for now.
>
> Finally, features such as xenaccess can lead to us trapping on
> arbitrary instructions accessing RAM and not just for MMIO. However in
> many such cases HSR.ISS is not valid and in general features such as
> xenaccess do not rely on the nature of the specific instruction, they
> resolve the fault (via information found elsewhere e.g. FAR_EL2)
> without needing to know anything about the instruction which triggered
> the trap.
>
> The register zr represents the zero register, i.e it will always
> return 0 and write to it is ignored. To properly handle this property,
> 2 new helpers have been introduced {get,set}_user_reg to read/write a
> value from/to a register. All the calls to select_user_reg have been
> replaced by these 2 helpers.
>
> Furthermore, the code to emulate encoding 31 in select_user_reg has been
> dropped because it was invalid. For Aarch64 context, the encoding is
> used for sp or zr. For AArch32 context, the ISS won't be valid for data
> abort from AArch32 using r15 (i.e pc) as source/destination (See D7-1881
> ARM DDI 0487A.d, note the validity is more restrictive than on ARMv7).
> It's also not possible to use r15 in co-processor instructions.
>
> This patch fixes setting MMIO register and sysreg to a random value
> (actually PC) instead of zero by something like:
>
> *((volatile int*)reg) = 0;
>
> compilers tend to generate "str wzr, [xx]" here.
>
> Reported-by: Marc Zyngier <Marc.Zyngier@arm.com>
> Signed-off-by: Julien Grall <julien.grall@citrix.com>
>
> Added BUG_ON to select_user_reg for {w,x}zr case.
>
> Clarified bits of the commit message by attempting to enumerate some
> of the cases which might be a concern in order to show why they are
> not.
>
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> ---
> v2: ijc: Took over the series, changes above.
I read the patch and looks good to me. You can add my
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
I have a couple of minor comments, which you can ignore or address as
you commit the patch.
> +register_t get_user_reg(struct cpu_user_regs *regs, int reg)
> +{
> +#ifdef CONFIG_ARM_64
> + /*
> + * For store/load and sysreg instruction, the encoding 31 always
> + * correspond to {w,x}zr which is the zero register.
> + */
> + if ( reg == 31 )
> + return 0;
> +#endif
> +
> + return *select_user_reg(regs, reg);
> +}
> +
> +void set_user_reg(struct cpu_user_regs *regs, int reg, register_t value)
> +{
> +#ifdef CONFIG_ARM_64
> + /*
> + * For store/load and sysreg instruction, the encoding 31 always
> + * correspond to {w,x}zr which is the zero register.
> + */
> + if ( reg == 31 )
> + return;
> +#endif
Couldn't we call a static inline function which is implemented
differently on CONFIG_ARM_64 and CONFIG_ARM_32 to do the reg == 31
check?
> + *select_user_reg(regs, reg) = value;
> +}
> +
> static const char *decode_fsc(uint32_t fsc, int *level)
> {
> const char *msg = NULL;
> @@ -1803,6 +1834,7 @@ static void do_cp15_32(struct cpu_user_regs *regs,
> inject_undef_exception(regs, hsr);
> return;
> }
> +
> advance_pc(regs, hsr);
> }
Spurious change
next prev parent reply other threads:[~2016-02-22 17:38 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-15 15:17 [PATCH v2] xen/arm64: correctly emulate the {w, x}zr registers Ian Campbell
2016-02-22 17:38 ` Stefano Stabellini [this message]
2016-03-23 18:10 ` Julien Grall
2016-03-24 10:54 ` Stefano Stabellini
2016-03-24 11:12 ` Julien Grall
2016-03-24 11:28 ` Jan Beulich
2016-03-24 11:49 ` Julien Grall
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