A fix is available
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
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
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.
[{"Business Unit":{"code":"BU054","label":"Systems w\/TPS"},"Product":{"code":"SG27M","label":"APARs - z\/VM environment"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"630","Edition":"","Line of Business":{"code":"LOB16","label":"Mainframe HW"}}]
Document Information
Modified date:
02 December 2016