linux-kernel - Re: [PATCH v6 2/2] kvm: sev: If ccp is busy, report busy to guest

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Message-ID: <b6b9b935-c5fa-dec6-ec82-56015b5dc733@amd.com>
Date: Thu, 5 Jun 2025 14:15:37 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Dionna Glaze <dionnaglaze@...gle.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: linux-coco@...ts.linux.dev, Paolo Bonzini <pbonzini@...hat.com>,
 Joerg Roedel <jroedel@...e.de>, Peter Gonda <pgonda@...gle.com>,
 Borislav Petkov <bp@...en8.de>, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v6 2/2] kvm: sev: If ccp is busy, report busy to guest

On 6/5/25 10:02, Dionna Glaze wrote:
> The ccp driver can be overloaded even with guest request rate limits.
> The return value of -EBUSY means that there is no firmware error to
> report back to user space, so the guest VM would see this as
> exitinfo2 = 0. The false success can trick the guest to update its
> message sequence number when it shouldn't have.

-EBUSY from the CCP driver is an error, not a throttling condition. Either
the driver has marked the ASP/PSP as dead or there are no command buffers
available, which is an error situation. There is no throttling support in
the CCP driver. A mutex is used to serialize requests, but all requests
proceed at some point. So there should not be a special check for -EBUSY.

Thanks,
Tom

> 
> Instead, when ccp returns -EBUSY, that is reported to userspace as the
> throttling return value.
> 
> Cc: Thomas Lendacky <Thomas.Lendacky@....com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Joerg Roedel <jroedel@...e.de>
> Cc: Peter Gonda <pgonda@...gle.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Sean Christopherson <seanjc@...gle.com>
> 
> Signed-off-by: Dionna Glaze <dionnaglaze@...gle.com>
> ---
>  arch/x86/kvm/svm/sev.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index e45f0cfae2bd..0ceb7e83a98d 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -4060,6 +4060,11 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
>  	 * the PSP is dead and commands are timing out.
>  	 */
>  	ret = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &fw_err);
> +	if (ret == -EBUSY) {
> +		svm_vmgexit_no_action(svm, SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_BUSY, fw_err));
> +		ret = 1;
> +		goto out_unlock;
> +	}
>  	if (ret && !fw_err)
>  		goto out_unlock;
>