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

Sean Christopherson
2022-07-21T00:38:33Z

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).