[pve-devel] [PATCH qemu-server v8 1/3] add C program to get hardware capabilities from CPUID

[Stefan Sterz]

On Thu Apr 25, 2024 at 1:24 PM CEST, Markus Frank wrote:
> Implement a systemd service that runs a C program that extracts AMD
> SEV hardware information such as reduced-phys-bios and cbitpos from
> CPUID at boot time, looks if SEV, SEV-ES & SEV-SNP are enabled, and
> outputs these details as JSON to /run/qemu-server/hw-params.json.
>
> This programm can also be used to read and save other hardware
> information at boot time.
>
> Signed-off-by: Markus Frank <m.frank at proxmox.com>
> Co-authored-by: Thomas Lamprecht <t.lamprecht at proxmox.com>
> ---
> v8:
> * renamed query-machine-params to query-machine-capabilities
>
> v7:
> * renamed amd-sev-support to query-machine-params
> * mv /run/amd-sev-params to /run/qemu-server/hw-params.json
> * add "mkdir /run/qemu-server" to ensure that the directory exists
> * moved json content to amd-sev property inside a bigger json
>  so that other hardware parameters could also be read at boot time and
>  included in this json file.
>
>  Makefile                                      |  1 +
>  query-machine-capabilities/Makefile           | 21 +++++++
>  .../query-machine-capabilities.c              | 55 +++++++++++++++++++
>  .../query-machine-capabilities.service        | 12 ++++
>  4 files changed, 89 insertions(+)
>  create mode 100644 query-machine-capabilities/Makefile
>  create mode 100644 query-machine-capabilities/query-machine-capabilities.c
>  create mode 100644 query-machine-capabilities/query-machine-capabilities.service
>
> diff --git a/Makefile b/Makefile
> index 133468d..ed67fe0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -65,6 +65,7 @@ install: $(PKGSOURCES)
>  	install -m 0644 -D bootsplash.jpg $(DESTDIR)/usr/share/$(PACKAGE)
>  	$(MAKE) -C PVE install
>  	$(MAKE) -C qmeventd install
> +	$(MAKE) -C query-machine-capabilities install
>  	$(MAKE) -C qemu-configs install
>  	$(MAKE) -C vm-network-scripts install
>  	install -m 0755 qm $(DESTDIR)$(SBINDIR)
> diff --git a/query-machine-capabilities/Makefile b/query-machine-capabilities/Makefile
> new file mode 100644
> index 0000000..c5f6348
> --- /dev/null
> +++ b/query-machine-capabilities/Makefile
> @@ -0,0 +1,21 @@
> +DESTDIR=
> +PREFIX=/usr
> +SBINDIR=${PREFIX}/libexec/qemu-server
> +SERVICEDIR=/lib/systemd/system
> +
> +CC ?= gcc
> +CFLAGS += -O2 -fanalyzer -Werror -Wall -Wextra -Wpedantic -Wtype-limits -Wl,-z,relro -std=gnu11
> +
> +query-machine-capabilities: query-machine-capabilities.c
> +	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS)
> +
> +.PHONY: install
> +install: query-machine-capabilities
> +	install -d ${DESTDIR}/${SBINDIR}
> +	install -d ${DESTDIR}${SERVICEDIR}
> +	install -m 0644 query-machine-capabilities.service ${DESTDIR}${SERVICEDIR}
> +	install -m 0755 query-machine-capabilities ${DESTDIR}${SBINDIR}
> +
> +.PHONY: clean
> +clean:
> +	rm -f query-machine-capabilities
> diff --git a/query-machine-capabilities/query-machine-capabilities.c b/query-machine-capabilities/query-machine-capabilities.c
> new file mode 100644
> index 0000000..f4a9f9f
> --- /dev/null
> +++ b/query-machine-capabilities/query-machine-capabilities.c
> @@ -0,0 +1,55 @@
> +#include <stdio.h>
> +#include <stdint.h>
> +#include <stdbool.h>
> +#include <sys/stat.h>
> +#include <sys/types.h>
> +
> +int main() {
> +    uint32_t eax, ebx, ecx, edx;
> +
> +    // query Encrypted Memory Capabilities, see:
> +    // https://en.wikipedia.org/wiki/CPUID#EAX=8000001Fh:_Encrypted_Memory_Capabilities
> +    uint32_t query_function = 0x8000001F;
> +    asm volatile("cpuid"
> +	 : "=a"(eax), "=b"(ebx), "=c"(ecx), "=d"(edx)
> +	 : "0"(query_function)
> +    );
> +
> +    bool sev_support = (eax & (1<<1)) != 0;
> +    bool sev_es_support = (eax & (1<<3)) != 0;
> +    bool sev_snp_support = (eax & (1<<4)) != 0;
> +
> +    uint8_t cbitpos = ebx & 0x3f;
> +    uint8_t reduced_phys_bits = (ebx >> 6) & 0x3f;
> +
> +    FILE *file;
> +    char filename[] = "/run/qemu-server/host-hw-capabilities.json";
> +
> +    mkdir("/run/qemu-server/", 0755);
> +

wouldn't it make sense to check whether this call succeeded too like you
do for the `fopen` below? also might be nice to use `strerror` and
handle `errno` in those cases too.

> +    file = fopen(filename, "w");
> +    if (file == NULL) {
> +	perror("Error opening file");
> +	return 1;
> +    }
> +
> +    fprintf(file,
> +	"{"
> +	" \"amd-sev\": {"
> +	" \"cbitpos\": %u,"
> +	" \"reduced-phys-bits\": %u,"
> +	" \"sev-support\": %s,"
> +	" \"sev-support-es\": %s,"
> +	" \"sev-support-snp\": %s"
> +	" }"
> +	" }\n",
> +	cbitpos,
> +	reduced_phys_bits,
> +	sev_support ? "true" : "false",
> +	sev_es_support ? "true" : "false",
> +	sev_snp_support ? "true" : "false"
> +    );
> +
> +    fclose(file);
> +    return 0;
> +}
> diff --git a/query-machine-capabilities/query-machine-capabilities.service b/query-machine-capabilities/query-machine-capabilities.service
> new file mode 100644
> index 0000000..f926074
> --- /dev/null
> +++ b/query-machine-capabilities/query-machine-capabilities.service
> @@ -0,0 +1,12 @@
> +[Unit]
> +Description=read AMD SEV parameters
> +RequiresMountsFor=/run
> +Before=pve-ha-lrm.service
> +Before=pve-guests.service
> +
> +[Service]
> +ExecStart=/usr/libexec/qemu-server/query-machine-capabilities
> +Type=oneshot
> +
> +[Install]
> +WantedBy=multi-user.target