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Message-ID: <CAAH4kHY_NyTD_uwA=EJ5U2RO7cCYJpdeBQ_enCj2ZjXVXAdLhA@mail.gmail.com>
Date: Wed, 14 May 2025 11:50:25 -0700
From: Dionna Amalie Glaze <dionnaglaze@...gle.com>
To: kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
Thomas Lendacky <Thomas.Lendacky@....com>, 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 v4 1/2] kvm: sev: Add SEV-SNP guest request throttling
On Wed, May 14, 2025 at 11:41 AM Dionna Glaze <dionnaglaze@...gle.com> wrote:
>
> The AMD-SP is a precious resource that doesn't have a scheduler other
> than a mutex lock queue. To avoid customers from causing a DoS, a
> mem_enc_ioctl command for rate limiting guest requests is added.
>
> Recommended values are {.interval_ms = 1000, .burst = 1} or
> {.interval_ms = 2000, .burst = 2} to average 1 request every second.
> You may need to allow 2 requests back to back to allow for the guest
> to query the certificate length in an extended guest request without
> a pause. The 1 second average is our target for quality of service
> since empirical tests show that 64 VMs can concurrently request an
> attestation report with a maximum latency of 1 second. We don't
> anticipate more concurrency than that for a seldom used request for
> a majority well-behaved set of VMs. The majority point is decided as
> >64 VMs given the assumed 128 VM count for "extreme load".
>
> 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>
> ---
> .../virt/kvm/x86/amd-memory-encryption.rst | 23 ++++++++++++++
> arch/x86/include/uapi/asm/kvm.h | 7 +++++
> arch/x86/kvm/svm/sev.c | 31 +++++++++++++++++++
> arch/x86/kvm/svm/svm.h | 2 ++
> 4 files changed, 63 insertions(+)
>
> diff --git a/Documentation/virt/kvm/x86/amd-memory-encryption.rst b/Documentation/virt/kvm/x86/amd-memory-encryption.rst
> index 1ddb6a86ce7f..1b5b4fc35aac 100644
> --- a/Documentation/virt/kvm/x86/amd-memory-encryption.rst
> +++ b/Documentation/virt/kvm/x86/amd-memory-encryption.rst
> @@ -572,6 +572,29 @@ Returns: 0 on success, -negative on error
> See SNP_LAUNCH_FINISH in the SEV-SNP specification [snp-fw-abi]_ for further
> details on the input parameters in ``struct kvm_sev_snp_launch_finish``.
>
> +21. KVM_SEV_SNP_SET_REQUEST_THROTTLE_RATE
> +-----------------------------------------
> +
> +The KVM_SEV_SNP_SET_REQUEST_THROTTLE_RATE command is used to set a per-VM rate
> +limit on responding to requests for AMD-SP to process a guest request.
> +The AMD-SP is a global resource with limited capacity, so to avoid noisy
> +neighbor effects, the host may set a request rate for guests.
> +
> +Parameters (in): struct kvm_sev_snp_set_request_throttle_rate
> +
> +Returns: 0 on success, -negative on error
> +
> +::
> +
> + struct kvm_sev_snp_set_request_throttle_rate {
> + __u32 interval_ms;
> + __u32 burst;
> + };
> +
> +The interval will be translated into jiffies, so if it after transformation
> +the interval is 0, the command will return ``-EINVAL``. The ``burst`` value
> +must be greater than 0.
> +
> Device attribute API
> ====================
>
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 460306b35a4b..d92242d9b9af 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -708,6 +708,8 @@ enum sev_cmd_id {
> KVM_SEV_SNP_LAUNCH_UPDATE,
> KVM_SEV_SNP_LAUNCH_FINISH,
>
> + KVM_SEV_SNP_SET_REQUEST_THROTTLE_RATE,
> +
> KVM_SEV_NR_MAX,
> };
>
> @@ -877,6 +879,11 @@ struct kvm_sev_snp_launch_finish {
> __u64 pad1[4];
> };
>
> +struct kvm_sev_snp_set_request_throttle_rate {
> + __u32 interval_ms;
> + __u32 burst;
> +};
> +
> #define KVM_X2APIC_API_USE_32BIT_IDS (1ULL << 0)
> #define KVM_X2APIC_API_DISABLE_BROADCAST_QUIRK (1ULL << 1)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index a7a7dc507336..febf4b45fddf 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2535,6 +2535,29 @@ static int snp_launch_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
> return ret;
> }
>
> +static int snp_set_request_throttle_ms(struct kvm *kvm, struct kvm_sev_cmd *argp)
> +{
> + struct kvm_sev_info *sev = to_kvm_sev_info(kvm);
> + struct kvm_sev_snp_set_request_throttle_rate params;
> + int ret;
> + u64 jiffies;
> +
> + if (!sev_snp_guest(kvm))
> + return -ENOTTY;
> +
> + if (copy_from_user(¶ms, u64_to_user_ptr(argp->data), sizeof(params)))
> + return -EFAULT;
> +
> + jiffies = (params.interval_ms * HZ) / 1000;
> +
> + if (!jiffies || !params.burst)
> + return -EINVAL;
> +
> + ratelimit_state_init(&sev->snp_guest_msg_rs, jiffies, params.burst);
> +
> + return 0;
> +}
> +
> int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
> {
> struct kvm_sev_cmd sev_cmd;
> @@ -2640,6 +2663,9 @@ int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
> case KVM_SEV_SNP_LAUNCH_FINISH:
> r = snp_launch_finish(kvm, &sev_cmd);
> break;
> + case KVM_SEV_SNP_SET_REQUEST_THROTTLE_RATE_MS:
> + r = snp_set_request_throttle_ms(kvm, &sev_cmd);
> + break;
> default:
> r = -EINVAL;
> goto out;
> @@ -4015,6 +4041,11 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
>
> mutex_lock(&sev->guest_req_mutex);
>
> + if (!__ratelimit(&sev->snp_guest_msg_rs)) {
> + rc = SNP_GUEST_VMM_ERR_BUSY;
embarrassing. My build totally skipped this error. Disregard as well.
> + goto out_unlock;
> + }
> +
> if (kvm_read_guest(kvm, req_gpa, sev->guest_req_buf, PAGE_SIZE)) {
> ret = -EIO;
> goto out_unlock;
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index f16b068c4228..0a7c8d3a7560 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -112,6 +112,8 @@ struct kvm_sev_info {
> void *guest_req_buf; /* Bounce buffer for SNP Guest Request input */
> void *guest_resp_buf; /* Bounce buffer for SNP Guest Request output */
> struct mutex guest_req_mutex; /* Must acquire before using bounce buffers */
> +
> + struct ratelimit_state snp_guest_msg_rs; /* Limit guest requests */
> };
>
> struct kvm_svm {
> --
> 2.49.0.1045.g170613ef41-goog
>
--
-Dionna Glaze, PhD, CISSP, CCSP (she/her)
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