Re: [Xen-devel] [V2 PATCH 0/9] x86/hvm: pkeys, add memory protection-key support

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/11/15 09:51, Huaitong Han wrote:
> Changes in v2:
> *Rebase all patches in staging branch
> *Disable X86_CR4_PKE on hypervisor, and delete pkru_read/write functions, and
> use xsave state read to get pkru value.
> *Delete the patch that adds pkeys support for do_page_fault.
> *Add pkeys support for gva2gfn so that setting _PAGE_PK_BIT in the return
> value can get propagated to the guest correctly.
>
> The protection-key feature provides an additional mechanism by which IA-32e
> paging controls access to usermode addresses.
>
> Hardware support for protection keys for user pages is enumerated with CPUID
> feature flag CPUID.7.0.ECX[3]:PKU. Software support is CPUID.7.0.ECX[4]:OSPKE
> with the setting of CR4.PKE(bit 22).
>
> When CR4.PKE = 1, every linear address is associated with the 4-bit protection
> key located in bits 62:59 of the paging-structure entry that mapped the page
> containing the linear address. The PKRU register determines, for each
> protection key, whether user-mode addresses with that protection key may be
> read or written.
>
> The PKRU register (protection key rights for user pages) is a 32-bit register
> with the following format: for each i (0 â i â 15), PKRU[2i] is the
> access-disable bit for protection key i (ADi); PKRU[2i+1] is the write-disable
> bit for protection key i (WDi).
>
> Software can use the RDPKRU and WRPKRU instructions with ECX = 0 to read and
> write PKRU. In addition, the PKRU register is XSAVE-managed state and can thus
> be read and written by instructions in the XSAVE feature set.
>
> PFEC.PK (bit 5) is defined as protection key violations.
>
> The specification of Protection Keys can be found at SDM (4.6.2, volume 3)
> http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf.

Just for my own understand, do you have a sample use-case for protection
keys?

As everything can WRPKRU, I cant see how it would actually be useful. 
Clearly there is a usecase or you (Intel) wouldn't have gone to the
effort of putting into silicon.

~Andrew

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