From: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx> Sent: Monday, January 23, 2023 8:51 AM
>
> The rmpupdate and psmash instructions, which are used in AMD's SEV-SNP
> to update the RMP (Reverse Map) table, can't be trapped. For nested
> scenarios, AMD defined MSR versions of these instructions which can be
s/can be/must be/ ??
> emulated by the top-level hypervisor. One instance where these MSRs are
And by "top-level", I think you are referring the hypervisor running at L1, right?
Using the L0/L1/L2 terminology would probably help make the description
more precise.
> used are Hyper-V VMs which expose SNP isolation features to the guest.
>
> The MSRs are defined in "AMD64 Architecture Programmer’s Manual, Volume 2:
> System Programming", section 15.36.19.
>
> Signed-off-by: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/cpufeatures.h | 1 +
> arch/x86/include/asm/msr-index.h | 2 +
> arch/x86/kernel/sev.c | 62 +++++++++++++++++++++++++-----
> 3 files changed, 55 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 480b4eaef310..e6e2e824f67b 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -423,6 +423,7 @@
> #define X86_FEATURE_SEV_SNP (19*32+ 4) /* AMD Secure Encrypted Virtualization - Secure Nested Paging */
> #define X86_FEATURE_V_TSC_AUX (19*32+ 9) /* "" Virtual TSC_AUX */
> #define X86_FEATURE_SME_COHERENT (19*32+10) /* "" AMD hardware-enforced cache coherency */
> +#define X86_FEATURE_NESTED_VIRT_SNP_MSR (19*32+29) /* Virtualizable RMPUPDATE and PSMASH MSR available */
>
> /*
> * BUG word(s)
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 35100c630617..d6103e607896 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -567,6 +567,8 @@
> #define MSR_AMD64_SEV_SNP_ENABLED
> BIT_ULL(MSR_AMD64_SEV_SNP_ENABLED_BIT)
> #define MSR_AMD64_RMP_BASE 0xc0010132
> #define MSR_AMD64_RMP_END 0xc0010133
> +#define MSR_AMD64_VIRT_RMPUPDATE 0xc001f001
> +#define MSR_AMD64_VIRT_PSMASH 0xc001f002
>
> #define MSR_AMD64_VIRT_SPEC_CTRL 0xc001011f
>
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 7fa39dc17edd..95404c7e5150 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2566,6 +2566,24 @@ int snp_lookup_rmpentry(u64 pfn, int *level)
> }
> EXPORT_SYMBOL_GPL(snp_lookup_rmpentry);
>
> +static bool virt_snp_msr(void)
> +{
> + return boot_cpu_has(X86_FEATURE_NESTED_VIRT_SNP_MSR);
> +}
> +
> +static u64 virt_psmash(u64 paddr)
> +{
> + int ret;
> +
> + asm volatile(
> + "wrmsr\n\t"
> + : "=a"(ret)
> + : "a"(paddr), "c"(MSR_AMD64_VIRT_PSMASH)
> + : "memory", "cc"
> + );
> + return ret;
> +}
>From checking the AMD spec, I can see that the above use
of wrmsr is non-conventional. Could you capture the basics
of the usage paradigm in a comment? I.e., the expected
inputs and outputs, and the core assumption that the
MSR isn't implemented in hardware, but must trap
to the hypervisor.
> +
> /*
> * psmash is used to smash a 2MB aligned page into 4K
> * pages while preserving the Validated bit in the RMP.
> @@ -2581,11 +2599,15 @@ int psmash(u64 pfn)
> if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> return -ENXIO;
>
> - /* Binutils version 2.36 supports the PSMASH mnemonic. */
> - asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> - : "=a"(ret)
> - : "a"(paddr)
> - : "memory", "cc");
> + if (virt_snp_msr()) {
> + ret = virt_psmash(paddr);
> + } else {
> + /* Binutils version 2.36 supports the PSMASH mnemonic. */
> + asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> + : "=a"(ret)
> + : "a"(paddr)
> + : "memory", "cc");
> + }
>
> return ret;
> }
> @@ -2601,6 +2623,21 @@ static int invalidate_direct_map(unsigned long pfn, int npages)
> return set_memory_np((unsigned long)pfn_to_kaddr(pfn), npages);
> }
>
> +static u64 virt_rmpupdate(unsigned long paddr, struct rmp_state *val)
> +{
> + int ret;
> + register u64 hi asm("r8") = ((u64 *)val)[1];
> + register u64 lo asm("rdx") = ((u64 *)val)[0];
> +
> + asm volatile(
> + "wrmsr\n\t"
> + : "=a"(ret)
> + : "a"(paddr), "c"(MSR_AMD64_VIRT_RMPUPDATE), "r"(lo), "r"(hi)
> + : "memory", "cc"
> + );
> + return ret;
> +}
Same here about a comment capturing the expected inputs
and outputs.
> +
> static int rmpupdate(u64 pfn, struct rmp_state *val)
> {
> unsigned long paddr = pfn << PAGE_SHIFT;
> @@ -2626,11 +2663,16 @@ static int rmpupdate(u64 pfn, struct rmp_state *val)
> }
>
> retry:
> - /* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> - asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
> - : "=a"(ret)
> - : "a"(paddr), "c"((unsigned long)val)
> - : "memory", "cc");
> +
> + if (virt_snp_msr()) {
> + ret = virt_rmpupdate(paddr, val);
> + } else {
> + /* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> + asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
> + : "=a"(ret)
> + : "a"(paddr), "c"((unsigned long)val)
> + : "memory", "cc");
> + }
>
> if (ret) {
> if (!retries) {
> --
> 2.25.1
