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KVM: SVM: Rewrite sev_es_prepare_switch_to_guest()'s comment about swap types

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Rewrite the comment(s) in sev_es_prepare_switch_to_guest() to explain the
swap types employed by the CPU for SEV-ES guests, i.e. to explain why KVM
needs to save a seemingly random subset of host state, and to provide a
decoder for the APM's Type-A/B/C terminology.

Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Link: https://lore.kernel.org/r/20230615063757.3039121-4-aik@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2023-06-15 16:37:51 +10:00
parent 29de732cc9
commit f8d808ed1b
1 changed files with 15 additions and 10 deletions
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25
arch/x86/kvm/svm/sev.c
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@ -3023,19 +3023,24 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm)
void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
{
/*
* As an SEV-ES guest, hardware will restore the host state on VMEXIT,
* of which one step is to perform a VMLOAD. KVM performs the
* corresponding VMSAVE in svm_prepare_guest_switch for both
* traditional and SEV-ES guests.
* All host state for SEV-ES guests is categorized into three swap types
* based on how it is handled by hardware during a world switch:
*
* A: VMRUN: Host state saved in host save area
* VMEXIT: Host state loaded from host save area
*
* B: VMRUN: Host state _NOT_ saved in host save area
* VMEXIT: Host state loaded from host save area
*
* C: VMRUN: Host state _NOT_ saved in host save area
* VMEXIT: Host state initialized to default(reset) values
*
* Manually save type-B state, i.e. state that is loaded by VMEXIT but
* isn't saved by VMRUN, that isn't already saved by VMSAVE (performed
* by common SVM code).
*/
/* XCR0 is restored on VMEXIT, save the current host value */
hostsa->xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
/* PKRU is restored on VMEXIT, save the current host value */
hostsa->pkru = read_pkru();
/* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
hostsa->xss = host_xss;
}