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Re: [Xen-devel] [PATCH v2 7/9] xen: arm: correctly handle sysreg accesses from userspace

On 19/02/15 12:23, Ian Campbell wrote:
> On Tue, 2015-02-17 at 15:25 +0000, Julien Grall wrote:
>> Hi Ian,
>> On 10/02/15 04:45, Ian Campbell wrote:
>>> Previously we implemented all registers as RAZ/WI even if they
>>> shouldn't be accessible to userspace.
>>> Accesses to the *_EL1 registers from EL0 are trapped to EL1 by the
>>> hardware, so add a BUG_ON. Likewise accesses from 32-bit EL1 cannot
>>> happen.
>>> PMUSERENR_EL0 and MDCCSR_EL0 are R/O to EL0.
>>> Other PM*_EL0 registers are accessible at EL0 only if
>>> PMUSERENR_EL0.EN is set, since we emulate that as RAZ/WI we know that
>>> bit cannot be set.
>> You assume that PMUSERENR_EL0.EN is always set to 0 during Xen boot.
>> Actually it's not the case and from the spec, it may be possible to have
>> it in an unkwnown state.
> The real PMUSERENR_EL0 is never seen/touched by EL1/EL0 since we trap
> all accesses (via MDCR_EL2.TPM) and emulate as RAZ, so the value of the
> real PMUSERENR_EL0 is never used (since MDCR_EL2 trumps it).

Hmmm... right sorry for the noise.


Julien Grall

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