linux-kernel - Re: [RFC PATCH v2 24/32] kvm: x86: prepare for SEV guest management API support

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Date:   Thu, 16 Mar 2017 11:33:03 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>, simon.guinot@...uanux.org,
        linux-efi@...r.kernel.org, kvm@...r.kernel.org, rkrcmar@...hat.com,
        matt@...eblueprint.co.uk, linux-pci@...r.kernel.org,
        linus.walleij@...aro.org, gary.hook@....com, linux-mm@...ck.org,
        paul.gortmaker@...driver.com, hpa@...or.com, cl@...ux.com,
        dan.j.williams@...el.com, aarcange@...hat.com,
        sfr@...b.auug.org.au, andriy.shevchenko@...ux.intel.com,
        herbert@...dor.apana.org.au, bhe@...hat.com, xemul@...allels.com,
        joro@...tes.org, x86@...nel.org, peterz@...radead.org,
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        akpm@...ux-foundation.org, davem@...emloft.net
Subject: Re: [RFC PATCH v2 24/32] kvm: x86: prepare for SEV guest management
 API support



On 02/03/2017 16:17, Brijesh Singh wrote:
> ASID management:
>  - Reserve asid range for SEV guest, SEV asid range is obtained through
>    CPUID Fn8000_001f[ECX]. A non-SEV guest can use any asid outside the SEV
>    asid range.

How is backwards compatibility handled?

>  - SEV guest must have asid value within asid range obtained through CPUID.
>  - SEV guest must have the same asid for all vcpu's. A TLB flush is required
>    if different vcpu for the same ASID is to be run on the same host CPU.

[...]

> +
> +	/* which host cpu was used for running this vcpu */
> +	bool last_cpuid;

Should be unsigned int.

> 
> +	/* Assign the asid allocated for this SEV guest */
> +	svm->vmcb->control.asid = asid;
> +
> +	/* Flush guest TLB:
> +	 * - when different VMCB for the same ASID is to be run on the
> +	 *   same host CPU
> +	 *   or
> +	 * - this VMCB was executed on different host cpu in previous VMRUNs.
> +	 */
> +	if (sd->sev_vmcbs[asid] != (void *)svm->vmcb ||

Why the cast?

> +		svm->last_cpuid != cpu)
> +		svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID;

If there is a match, you don't need to do anything else (neither reset
the asid, nor mark it as dirty, nor update the fields), so:

	if (sd->sev_vmcbs[asid] == svm->vmcb &&
	    svm->last_cpuid == cpu)
		return;

	svm->last_cpuid = cpu;
	sd->sev_vmcbs[asid] = svm->vmcb;
	svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID;
	svm->vmcb->control.asid = asid;
	mark_dirty(svm->vmcb, VMCB_ASID);

(plus comments ;)).

Also, why not TLB_CONTROL_FLUSH_ASID if possible?

> +	svm->last_cpuid = cpu;
> +	sd->sev_vmcbs[asid] = (void *)svm->vmcb;
> +
> +	mark_dirty(svm->vmcb, VMCB_ASID);

[...]

> 
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index fef7d83..9df37a2 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1284,6 +1284,104 @@ struct kvm_s390_ucas_mapping {
>  /* Memory Encryption Commands */
>  #define KVM_MEMORY_ENCRYPT_OP	  _IOWR(KVMIO, 0xb8, unsigned long)
>  
> +/* Secure Encrypted Virtualization mode */
> +enum sev_cmd_id {

Please add documentation in Documentation/virtual/kvm/memory_encrypt.txt.

Paolo