Re: [PATCH v2 09/11] KVM: x86: emulator/smm: use smram struct for 64 bit smram load/restore

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 21, 2022, Maxim Levitsky wrote:
> Use kvm_smram_state_64 struct to save/restore the 64 bit SMM state
> (used when X86_FEATURE_LM is present in the guest CPUID,
> regardless of 32-bitness of the guest).
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
> @@ -9814,7 +9805,7 @@ static void enter_smm(struct kvm_vcpu *vcpu)
>  	memset(buf, 0, 512);
>  #ifdef CONFIG_X86_64
>  	if (guest_cpuid_has(vcpu, X86_FEATURE_LM))
> -		enter_smm_save_state_64(vcpu, buf);
> +		enter_smm_save_state_64(vcpu, (struct kvm_smram_state_64 *)buf);
>  	else
>  #endif
>  		enter_smm_save_state_32(vcpu, (struct kvm_smram_state_32 *)buf);

Hrm, I _love_ the approach overall, but I really dislike having to cast an
arbitrary buffer, especially in the SVM code.

Aha!  Rather than keeping a buffer and casting, create a union to hold everything:

	union kvm_smram {
		struct kvm_smram_state_64 smram64;
		struct kvm_smram_state_32 smram32;
		u8 bytes[512];
	};

and then enter_smm() becomes:

  static void enter_smm(struct kvm_vcpu *vcpu)
  {
	struct kvm_segment cs, ds;
	struct desc_ptr dt;
	unsigned long cr0;

	union kvm_smram smram;

	BUILD_BUG_ON(sizeof(smram) != 512);

	memset(smram.bytes, 0, sizeof(smram));
#ifdef CONFIG_X86_64
	if (guest_cpuid_has(vcpu, X86_FEATURE_LM))
		enter_smm_save_state_64(vcpu, &smram.smram64);
	else
#endif
		enter_smm_save_state_32(vcpu, &smram.smram32);

	/*
	 * Give enter_smm() a chance to make ISA-specific changes to the vCPU
	 * state (e.g. leave guest mode) after we've saved the state into the
	 * SMM state-save area.
	 */
	static_call(kvm_x86_enter_smm)(vcpu, &smram);

	kvm_smm_changed(vcpu, true);
	kvm_vcpu_write_guest(vcpu, vcpu->arch.smbase + 0xfe00, smram.bytes, sizeof(smram));

and em_rsm() gets similar treatment.  Then the vendor code doesn't have to cast,
e.g. SVM can do:

	smram->smram64.svm_guest_flag = 1;
	smram->smram64.svm_guest_vmcb_gpa = svm->nested.vmcb12_gpa;

That way we don't have to refactor this all again if we want to use SMRAM to save
something on Intel for VMX (though I agree with Jim that that's probably a bad idea).