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Message-ID: <YtigALu4ccTi4/v0@google.com>
Date: Thu, 21 Jul 2022 00:38:24 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, x86@...nel.org,
Kees Cook <keescook@...omium.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
Ingo Molnar <mingo@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 09/11] KVM: x86: emulator/smm: use smram struct for 64
bit smram load/restore
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@...hat.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).
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