Re: [PATCH v2 4/8] KVM: SVM: retrieve VMCB from assembly

[Sean Christopherson]

 On Tue, Nov 08, 2022, Paolo Bonzini wrote:
> This is needed in order to keep the number of arguments to 3 or less,
> after adding hsave_pa and spec_ctrl_intercepted.  32-bit builds only
> support passing three arguments in registers, fortunately all other
> data is reachable from the vcpu_svm struct.

Is it actually a problem if parameters are passed on the stack?  The assembly
code mostly creates a stack frame, i.e. %ebp can be used to pull values off the
stack.

I dont think it will matter in the end (more below), but hypothetically if we
ended up with

  void __svm_vcpu_run(struct kvm_vcpu *vcpu, unsigned long vmcb_pa,
		      unsigned long gsave_pa, unsigned long hsave_pa,
		      bool spec_ctrl_intercepted);

then the asm prologue would be something like:

	/*
	 * Save @vcpu, @gsave_pa, @hsave_pa, and @spec_ctrl_intercepted, all of
	 * which are needed after VM-Exit.
	 */
	push %_ASM_ARG1
	push %_ASM_ARG3

  #ifdef CONFIG_X86_64
	push %_ASM_ARG4
	push %_ASM_ARG5
  #else
	push %_ASM_ARG4_EBP(%ebp)
	push %_ASM_ARG5_EBP(%ebp)
  #endif

which is a few extra memory accesses, especially for 32-bit, but no one cares
about 32-bit and I highly doubt a few extra PUSH+POP instructions will be noticeable.

Threading in yesterday's conversation...

> > What about adding dedicated structs to hold the non-regs params for VM-Enter and
> > VMRUN?  Grabbing stuff willy-nilly in the assembly code makes the flows difficult
> > to read as there's nothing in the C code that describes what fields are actually
> > used.
>
> What fields are actually used is (like with any other function)
> "potentially all, you'll have to read the source code and in fact you
> can just read asm-offsets.c instead".  What I mean is, I cannot offhand
> see or remember what fields are touched by svm_prepare_switch_to_guest,
> why would __svm_vcpu_run be any different?

It's different because if it were a normal C function, it would simply take
@vcpu, and maybe @spec_ctrl_intercepted to shave cycles after CLGI.  But because
it's assembly and doesn't have to_svm() readily available (among other restrictions),
__svm_vcpu_run() ends up taking a mishmash of parameters, which for me makes it
rather difficult to understand what to expect.

Oooh, and after much staring I realized that the address of the host save area
is passed in because grabbing it after VM-Exit can't work.  That's subtle, and
passing it in isn't strictly necessary; there's no reason the assembly code can't
grab it and stash it on the stack.

What about killing a few birds with one stone?  Move the host save area PA to
its own per-CPU variable, and then grab that from assembly as well.  Then it's
a bit more obvious why the address needs to be saved on the stack across VMRUN,
and my whining about the prototype being funky goes away.  __svm_vcpu_run() and
__svm_sev_es_vcpu_run() would have identical prototypes too.

Attached patches would slot in early in the series.  Tested SVM and SME-enabled
kernels, didn't test the SEV-ES bits.