[pve-devel] [PATCH qemu-server v6 1/3] add C program to get AMD SEV hardware parameters from CPUID

[Thomas Lamprecht]

Am 19/04/2024 um 12:59 schrieb Markus Frank:
> Implement a systemd service that runs a C program that extracts AMD SEV
> hardware parameters 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/amd-sev-params.

general comments:

I'd not call this SEV support, we will probably re-use this to get other
features from both Intel and AMD in the future, so I'd name it something
slightly more general like "query-machine-capabilities"

The output file should not pollute top-level /run, we already have
/run/qemu-server in use, so that could be used here too.

> 

misses a co-authored or originally-by me trailer ;-)

> Signed-off-by: Markus Frank <m.frank at proxmox.com>
> ---
>  Makefile                                |  1 +
>  amd-sev-support/Makefile                | 21 +++++++++++
>  amd-sev-support/amd-sev-support.c       | 48 +++++++++++++++++++++++++
>  amd-sev-support/amd-sev-support.service | 12 +++++++
>  4 files changed, 82 insertions(+)
>  create mode 100644 amd-sev-support/Makefile
>  create mode 100644 amd-sev-support/amd-sev-support.c
>  create mode 100644 amd-sev-support/amd-sev-support.service
> 
> diff --git a/Makefile b/Makefile
> index 133468d..ccd12a1 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 amd-sev-support install
>  	$(MAKE) -C qemu-configs install
>  	$(MAKE) -C vm-network-scripts install
>  	install -m 0755 qm $(DESTDIR)$(SBINDIR)
> diff --git a/amd-sev-support/Makefile b/amd-sev-support/Makefile
> new file mode 100644
> index 0000000..022ed94
> --- /dev/null
> +++ b/amd-sev-support/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
> +
> +amd-sev-support: amd-sev-support.c
> +	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS)
> +
> +.PHONY: install
> +install: amd-sev-support
> +	install -d ${DESTDIR}/${SBINDIR}
> +	install -d ${DESTDIR}${SERVICEDIR}
> +	install -m 0644 amd-sev-support.service ${DESTDIR}${SERVICEDIR}
> +	install -m 0755 amd-sev-support ${DESTDIR}${SBINDIR}
> +
> +.PHONY: clean
> +clean:
> +	rm -f amd-sev-support
> diff --git a/amd-sev-support/amd-sev-support.c b/amd-sev-support/amd-sev-support.c
> new file mode 100644
> index 0000000..73a7bd8
> --- /dev/null
> +++ b/amd-sev-support/amd-sev-support.c
> @@ -0,0 +1,48 @@
> +#include <stdio.h>
> +#include <stdint.h>
> +#include <stdbool.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/amd-sev-params";

besides saving this in the /run/qemu-server directory it'd be additionally
nice to add a ".json" file type suffix to clarify what format this
is in.

> +
> +    file = fopen(filename, "w");
> +    if (file == NULL) {
> +	perror("Error opening file");
> +	return 1;
> +    }
> +
> +    fprintf(file, "{"
> +	" \"cbitpos\": %u,"
> +	" \"reduced-phys-bits\": %u,"
> +	" \"sev\": %s,"
> +	" \"sev-es\": %s,"
> +	" \"sev-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/amd-sev-support/amd-sev-support.service b/amd-sev-support/amd-sev-support.service
> new file mode 100644
> index 0000000..466dd0a
> --- /dev/null
> +++ b/amd-sev-support/amd-sev-support.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/amd-sev-support
> +Type=forking

why is this "forking", you do not even fork once?! (nor should you)
I'd use the "oneshot" type that is designed for such use cases.
 

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