Transitioning to/from a VMX guest requires KVM to manually save/load
the bulk of CPU state that the guest is allowed to direclty access,
e.g. XSAVE state, CR2, GPRs, etc... For obvious reasons, loading the
guest's GPR snapshot prior to VM-Enter and saving the snapshot after
VM-Exit is done via handcoded assembly. The assembly blob is written
as inline asm so that it can easily access KVM-defined structs that
are used to hold guest state, e.g. moving the blob to a standalone
assembly file would require generating defines for struct offsets.
The other relevant aspect of VMX transitions in KVM is the handling of
VM-Exits. KVM doesn't employ a separate VM-Exit handler per se, but
rather treats the VMX transition as a mega instruction (with many side
effects), i.e. sets the VMCS.HOST_RIP to a label immediately following
VMLAUNCH/VMRESUME. The label is then exposed to C code via a global
variable definition in the inline assembly.
Because of the global variable, KVM takes steps to (attempt to) ensure
only a single instance of the owning C function, e.g. vmx_vcpu_run, is
generated by the compiler. The earliest approach placed the inline
assembly in a separate noinline function[1]. Later, the assembly was
folded back into vmx_vcpu_run() and tagged with __noclone[2][3], which
is still used today.
After moving to __noclone, an edge case was encountered where GCC's
-ftracer optimization resulted in the inline assembly blob being
duplicated. This was "fixed" by explicitly disabling -ftracer in the
__noclone definition[4].
Recently, it was found that disabling -ftracer causes build warnings
for unsuspecting users of __noclone[5], and more importantly for KVM,
prevents the compiler for properly optimizing vmx_vcpu_run()[6]. And
perhaps most importantly of all, it was pointed out that there is no
way to prevent duplication of a function with 100% reliability[7],
i.e. more edge cases may be encountered in the future.
So to summarize, the only way to prevent the compiler from duplicating
the global variable definition is to move the variable out of inline
assembly, which has been suggested several times over[1][7][8].
Resolve the aforementioned issues by moving the VMLAUNCH+VRESUME and
VM-Exit "handler" to standalone assembly sub-routines. Moving only
the core VMX transition codes allows the struct indexing to remain as
inline assembly and also allows the sub-routines to be used by
nested_vmx_check_vmentry_hw(). Reusing the sub-routines has a happy
side-effect of eliminating two VMWRITEs in the nested_early_check path
as there is no longer a need to dynamically change VMCS.HOST_RIP.
Note that callers to vmx_vmenter() must account for the CALL modifying
RSP, e.g. must subtract op-size from RSP when synchronizing RSP with
VMCS.HOST_RSP and "restore" RSP prior to the CALL. There are no great
alternatives to fudging RSP. Saving RSP in vmx_enter() is difficult
because doing so requires a second register (VMWRITE does not provide
an immediate encoding for the VMCS field and KVM supports Hyper-V's
memory-based eVMCS ABI). The other more drastic alternative would be
to use eschew VMCS.HOST_RSP and manually save/load RSP using a per-cpu
variable (which can be encoded as e.g. gs:[imm]). But because a valid
stack is needed at the time of VM-Exit (NMIs aren't blocked and a user
could theoretically insert INT3/INT1ICEBRK at the VM-Exit handler), a
dedicated per-cpu VM-Exit stack would be required. A dedicated stack
isn't difficult to implement, but it would require at least one page
per CPU and knowledge of the stack in the dumpstack routines. And in
most cases there is essentially zero overhead in dynamically updating
VMCS.HOST_RSP, e.g. the VMWRITE can be avoided for all but the first
VMLAUNCH unless nested_early_check=1, which is not a fast path. In
other words, avoiding the VMCS.HOST_RSP by using a dedicated stack
would only make the code marginally less ugly while requiring at least
one page per CPU and forcing the kernel to be aware (and approve) of
the VM-Exit stack shenanigans.
[1] cea15c24ca39 ("KVM: Move KVM context switch into own function")
[2] a3b5ba49a8 ("KVM: VMX: add the __noclone attribute to vmx_vcpu_run")
[3] 104f226bfd ("KVM: VMX: Fold __vmx_vcpu_run() into vmx_vcpu_run()")
[4] 95272c2937 ("compiler-gcc: disable -ftracer for __noclone functions")
[5] https://lkml.kernel.org/r/20181218140105.ajuiglkpvstt3qxs@treble
[6] https://patchwork.kernel.org/patch/8707981/#21817015
[7] https://lkml.kernel.org/r/ri6y38lo23g.fsf@suse.cz
[8] https://lkml.kernel.org/r/20181218212042.GE25620@tassilo.jf.intel.com
Suggested-by: Andi Kleen <ak@linux.intel.com>
Suggested-by: Martin Jambor <mjambor@suse.cz>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Martin Jambor <mjambor@suse.cz>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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