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VM64941: STORAGE KEY CHANGE-BIT OVER-INDICATION

A fix is available

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APAR status

  • Closed as program error.

Error description

  • Following a guest's reset of the change-bit in a frame's storage
    key to '0'b, the guest observes a change-bit of '1'b.
    

Local fix

Problem summary

  • ****************************************************************
    * USERS AFFECTED: z/VM guest operating systems who use the     *
    *                 change-bit in the storage key                *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    ****************************************************************
    * RECOMMENDATION: APPLY PTF                                    *
    ****************************************************************
    z/VM over-indicates the change-bit in a frame's storage key
    in the following scenario:
    1. In preparation for page-steal, z/VM puts a guest page into
       the invalid-but-resident (IBR) state.  When a page is IBR
       the Page Table Entry's (PTE) state is invalid.  However, the
       guest's page content remains in the frame.
    2. While in the IBR state, the guest operating system issues
       a SSKE instruction to reset the change-bit in the storage
       key.  Because the PTE is invalid, only the guest's view of
       the storage key in the Page Table Status Entry (PGSTE) is
       updated.  The actual storage key for the page-frame is
       unchanged.
    3. If the guest subsequently fetches an address within the
       page (which removes it from the IBR state), followed by an
       ISKE instruction to interrogate the storage key, the
       value of the change-bit within the storage key returned by
       ISKE is '1'b when it should be '0'b because of the SSKE
       performed in step 2.
    
    This misrepresentation of the change-bit might cause a guest
    operating system to perform additional processing that would
    otherwise be unnecessary.
    

Problem conclusion

  • The data preserved when a page is made IBR has been extended
    to include the reference and change bits in the frame's storage
    key.  Upon revalidation of an IBR page the preserved storage
    key is compared with the guest's view of the storage key in
    the PGSTE.  If the values differ, then z/VM issues a SSKE to
    update the frame's storage key to match the guest's view in the
    PGSTE.
    

Temporary fix

  • FOR RELEASE VM/ESA CP/ESA R630 :
    PREREQ: VM65586 VM65368
    CO-REQ: NONE
    IF-REQ: NONE
    

Comments

APAR Information

  • APAR number

    VM64941

  • Reported component name

    VM CP

  • Reported component ID

    568411202

  • Reported release

    630

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    YesHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2015-09-17

  • Closed date

    2015-12-16

  • Last modified date

    2016-12-02

  • APAR is sysrouted FROM one or more of the following:

  • APAR is sysrouted TO one or more of the following:

    UM34584

Modules/Macros

  • HCPALD   HCPALG   HCPFRMTE HCPHTT   HCPPFR
    HCPPTA   HCPPTC
    

Fix information

  • Fixed component name

    VM CP

  • Fixed component ID

    568411202

Applicable component levels

  • R630 PSY UM34584

       UP15/12/18 P 1602 Ž

Fix is available

  • Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.

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Document Information

Modified date:
02 December 2016