smbios: cap DIMM size to 2Tb as workaround for broken Windows

With current limit set to match max spec size (2PTb),
Windows fails to parse type 17 records when DIMM size reaches 4Tb+.
Failure happens in GetPhysicallyInstalledSystemMemory() function,
and fails "Check SMBIOS System Memory Tables" SVVP test.
Though not fatal, it might cause issues for userspace apps,
something like [1].

Lets cap default DIMM size to 2Tb for now, until MS fixes it.

1) https://issues.redhat.com/browse/RHEL-81999?focusedId=27731200&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-27731200

PS: It's obvious 32 int overflow math somewhere in Windows,
    MS admitted that it's Windows bug and in a process of fixing it.
    However it's unclear if W10 and earlier would get the fix.
    So however I dislike changing defaults, we heed to work around
    the issue (it looks like QEMU regression while not being it).
    Hopefully 2Tb/DIMM split will last longer until VM memory size
    will become large enough to cause to many type 17 records issue
    again.
PPS:
    Alternatively, instead of messing with defaults, we can create
    a dedicated knob to ask for desired DIMM size cap explicitly
    on CLI. That will let users to enable workaround when they
    hit this corner case. Downside is that knob has to be propagated
    up all mgmt stack, which might be not desirable.
PPPS:
    Yet alternatively, users can configure initial RAM to be less
    than 4Tb and all additional RAM add as DIMMs on QEMU CLI.
    (however it's the job to be done by mgmt which could know
    Windows version and total amount of RAM)

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Fixes: 62f182c97b ("smbios: make memory device size configurable per Machine")
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20250901084915.2607632-1-imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

0 files changed, 0 insertions, 0 deletions