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05243d490b
Move FSGSBASE enablement from identify_cpu() to cpu_init_exception_handling()
to ensure it is enabled before any exceptions can occur on both boot and
secondary CPUs.
== Background ==
Exception entry code (paranoid_entry()) uses ALTERNATIVE patching based on
X86_FEATURE_FSGSBASE to decide whether to use RDGSBASE/WRGSBASE instructions
or the slower RDMSR/SWAPGS sequence for saving/restoring GSBASE.
On boot CPU, ALTERNATIVE patching happens after enabling FSGSBASE in CR4.
When the feature is available, the code is permanently patched to use
RDGSBASE/WRGSBASE, which require CR4.FSGSBASE=1 to execute without triggering
== Boot Sequence ==
Boot CPU (with CR pinning enabled):
trap_init()
cpu_init() <- Uses unpatched code (RDMSR/SWAPGS)
x2apic_setup()
...
arch_cpu_finalize_init()
identify_boot_cpu()
identify_cpu()
cr4_set_bits(X86_CR4_FSGSBASE) # Enables the feature
# This becomes part of cr4_pinned_bits
...
alternative_instructions() <- Patches code to use RDGSBASE/WRGSBASE
Secondary CPUs (with CR pinning enabled):
start_secondary()
cr4_init() <- Code already patched, CR4.FSGSBASE=1
set implicitly via cr4_pinned_bits
cpu_init() <- exceptions work because FSGSBASE is
already enabled
Secondary CPU (with CR pinning disabled):
start_secondary()
cr4_init() <- Code already patched, CR4.FSGSBASE=0
cpu_init()
x2apic_setup()
rdmsrq(MSR_IA32_APICBASE) <- Triggers #VC in SNP guests
exc_vmm_communication()
paranoid_entry() <- Uses RDGSBASE with CR4.FSGSBASE=0
(patched code)
...
ap_starting()
identify_secondary_cpu()
identify_cpu()
cr4_set_bits(X86_CR4_FSGSBASE) <- Enables the feature, which is
too late
== CR Pinning ==
Currently, for secondary CPUs, CR4.FSGSBASE is set implicitly through
CR-pinning: the boot CPU sets it during identify_cpu(), it becomes part of
cr4_pinned_bits, and cr4_init() applies those pinned bits to secondary CPUs.
This works but creates an undocumented dependency between cr4_init() and the
pinning mechanism.
== Problem ==
Secondary CPUs boot after alternatives have been applied globally. They
execute already-patched paranoid_entry() code that uses RDGSBASE/WRGSBASE
instructions, which require CR4.FSGSBASE=1. Upcoming changes to CR pinning
behavior will break the implicit dependency, causing secondary CPUs to
generate #UD.
This issue manifests itself on AMD SEV-SNP guests, where the rdmsrq() in
x2apic_setup() triggers a #VC exception early during cpu_init(). The #VC
handler (exc_vmm_communication()) executes the patched paranoid_entry() path.
Without CR4.FSGSBASE enabled, RDGSBASE instructions trigger #UD.
== Fix ==
Enable FSGSBASE explicitly in cpu_init_exception_handling() before loading
exception handlers. This makes the dependency explicit and ensures both
boot and secondary CPUs have FSGSBASE enabled before paranoid_entry()
executes.
Fixes:
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