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Message-ID: <CAMj1kXF-ufGkWRkEOwOLntPOuVS1skLJ8HA0FpfJCQbvoDeTJA@mail.gmail.com>
Date: Thu, 10 Jul 2025 09:21:19 +1000
From: Ard Biesheuvel <ardb@...nel.org>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
linux-efi@...r.kernel.org, x86@...nel.org, Borislav Petkov <bp@...en8.de>,
Ingo Molnar <mingo@...nel.org>, Dionna Amalie Glaze <dionnaglaze@...gle.com>,
Kevin Loughlin <kevinloughlin@...gle.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 01/24] x86/sev: Separate MSR and GHCB based snp_cpuid()
via a callback
On Thu, 10 Jul 2025 at 01:12, Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 7/9/25 03:08, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > There are two distinct callers of snp_cpuid(): one where the MSR
> > protocol is always used, and one where the GHCB page based interface is
> > always used.
> >
> > The snp_cpuid() logic does not care about the distinction, which only
> > matters at a lower level. But the fact that it supports both interfaces
> > means that the GHCB page based logic is pulled into the early startup
> > code where PA to VA conversions are problematic, given that it runs from
> > the 1:1 mapping of memory.
> >
> > So keep snp_cpuid() itself in the startup code, but factor out the
> > hypervisor calls via a callback, so that the GHCB page handling can be
> > moved out.
> >
> > Code refactoring only - no functional change intended.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> > arch/x86/boot/startup/sev-shared.c | 64 ++++----------------
> > arch/x86/coco/sev/vc-shared.c | 49 ++++++++++++++-
> > arch/x86/include/asm/sev.h | 3 +-
> > 3 files changed, 63 insertions(+), 53 deletions(-)
> >
> > diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
> > index 7a706db87b93..992abfa50508 100644
> > --- a/arch/x86/boot/startup/sev-shared.c
> > +++ b/arch/x86/boot/startup/sev-shared.c
> > @@ -319,7 +319,7 @@ static int __sev_cpuid_hv(u32 fn, int reg_idx, u32 *reg)
> > return 0;
> > }
> >
> > -static int __sev_cpuid_hv_msr(struct cpuid_leaf *leaf)
> > +static int __sev_cpuid_msr_prot(struct cpuid_leaf *leaf)
>
> Not sure the renaming makes it read any easier or say anything more. It
> does add extra changes to the diff that have to be read through, though,
> so I don't think it is beneficial.
>
These additional changes were provided by Boris as a delta patch on
top of my v3, so I will leave to him to respond to this.
https://lore.kernel.org/all/20250512190834.332684-24-ardb+git@google.com/T/#u
> > {
> > int ret;
> >
> > @@ -342,44 +342,7 @@ static int __sev_cpuid_hv_msr(struct cpuid_leaf *leaf)
> > return ret;
> > }
> >
> > -static int __sev_cpuid_hv_ghcb(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
> > -{
> > - u32 cr4 = native_read_cr4();
> > - int ret;
> > -
> > - ghcb_set_rax(ghcb, leaf->fn);
> > - ghcb_set_rcx(ghcb, leaf->subfn);
> > -
> > - if (cr4 & X86_CR4_OSXSAVE)
> > - /* Safe to read xcr0 */
> > - ghcb_set_xcr0(ghcb, xgetbv(XCR_XFEATURE_ENABLED_MASK));
> > - else
> > - /* xgetbv will cause #UD - use reset value for xcr0 */
> > - ghcb_set_xcr0(ghcb, 1);
> > -
> > - ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
> > - if (ret != ES_OK)
> > - return ret;
> > -
> > - if (!(ghcb_rax_is_valid(ghcb) &&
> > - ghcb_rbx_is_valid(ghcb) &&
> > - ghcb_rcx_is_valid(ghcb) &&
> > - ghcb_rdx_is_valid(ghcb)))
> > - return ES_VMM_ERROR;
> >
> > - leaf->eax = ghcb->save.rax;
> > - leaf->ebx = ghcb->save.rbx;
> > - leaf->ecx = ghcb->save.rcx;
> > - leaf->edx = ghcb->save.rdx;
> > -
> > - return ES_OK;
> > -}
> > -
> > -static int sev_cpuid_hv(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
> > -{
> > - return ghcb ? __sev_cpuid_hv_ghcb(ghcb, ctxt, leaf)
> > - : __sev_cpuid_hv_msr(leaf);
> > -}
> >
> > /*
> > * This may be called early while still running on the initial identity
> > @@ -484,21 +447,20 @@ snp_cpuid_get_validated_func(struct cpuid_leaf *leaf)
> > return false;
> > }
> >
> > -static void snp_cpuid_hv(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
> > +static void snp_cpuid_msr_prot(void *ctx, struct cpuid_leaf *leaf)
> > {
> > - if (sev_cpuid_hv(ghcb, ctxt, leaf))
> > + if (__sev_cpuid_msr_prot(leaf))
> > sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_CPUID_HV);
> > }
> >
> > -static int __head
> > -snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
> > - struct cpuid_leaf *leaf)
> > +static int __head snp_cpuid_postprocess(void (*cpuid)(void *ctx, struct cpuid_leaf *),
> > + void *ctx, struct cpuid_leaf *leaf)
> > {
> > struct cpuid_leaf leaf_hv = *leaf;
> >
> > switch (leaf->fn) {
> > case 0x1:
> > - snp_cpuid_hv(ghcb, ctxt, &leaf_hv);
> > + cpuid(ctx, &leaf_hv);
>
> Maybe rename this parameter to snp_cpuid or snp_cpuid_fn or similar,
> because it can be very confusing to see "cpuid" on its own like this.
>
> >
> > /* initial APIC ID */
> > leaf->ebx = (leaf_hv.ebx & GENMASK(31, 24)) | (leaf->ebx & GENMASK(23, 0));
> > @@ -517,7 +479,7 @@ snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
> > break;
> > case 0xB:
> > leaf_hv.subfn = 0;
> > - snp_cpuid_hv(ghcb, ctxt, &leaf_hv);
> > + cpuid(ctx, &leaf_hv);
> >
> > /* extended APIC ID */
> > leaf->edx = leaf_hv.edx;
> > @@ -565,7 +527,7 @@ snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
> > }
> > break;
> > case 0x8000001E:
> > - snp_cpuid_hv(ghcb, ctxt, &leaf_hv);
> > + cpuid(ctx, &leaf_hv);
> >
> > /* extended APIC ID */
> > leaf->eax = leaf_hv.eax;
> > @@ -586,8 +548,8 @@ snp_cpuid_postprocess(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
> > * Returns -EOPNOTSUPP if feature not enabled. Any other non-zero return value
> > * should be treated as fatal by caller.
> > */
> > -int __head
> > -snp_cpuid(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
> > +int __head snp_cpuid(void (*cpuid)(void *ctx, struct cpuid_leaf *), void *ctx,
> > + struct cpuid_leaf *leaf)
> > {
> > const struct snp_cpuid_table *cpuid_table = snp_cpuid_get_table();
> >
> > @@ -621,7 +583,7 @@ snp_cpuid(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
> > return 0;
> > }
> >
> > - return snp_cpuid_postprocess(ghcb, ctxt, leaf);
> > + return snp_cpuid_postprocess(cpuid, ctx, leaf);
> > }
> >
> > /*
> > @@ -648,14 +610,14 @@ void __head do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
> > leaf.fn = fn;
> > leaf.subfn = subfn;
> >
> > - ret = snp_cpuid(NULL, NULL, &leaf);
> > + ret = snp_cpuid(snp_cpuid_msr_prot, NULL, &leaf);
> > if (!ret)
> > goto cpuid_done;
> >
> > if (ret != -EOPNOTSUPP)
> > goto fail;
> >
> > - if (__sev_cpuid_hv_msr(&leaf))
> > + if (__sev_cpuid_msr_prot(&leaf))
> > goto fail;
> >
> > cpuid_done:
> > diff --git a/arch/x86/coco/sev/vc-shared.c b/arch/x86/coco/sev/vc-shared.c
> > index 2c0ab0fdc060..776cb90be530 100644
> > --- a/arch/x86/coco/sev/vc-shared.c
> > +++ b/arch/x86/coco/sev/vc-shared.c
> > @@ -409,15 +409,62 @@ static enum es_result vc_handle_ioio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
> > return ret;
> > }
> >
> > +static int __sev_cpuid_ghcb_prot(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
>
> Ditto here and below, keeping the __sev_cpuid_hv_ghcb() /
> sev_cpuid_hv_ghcb() name would be best.
>
> Thanks,
> Tom
>
> > +{
> > + u32 cr4 = native_read_cr4();
> > + int ret;
> > +
> > + ghcb_set_rax(ghcb, leaf->fn);
> > + ghcb_set_rcx(ghcb, leaf->subfn);
> > +
> > + if (cr4 & X86_CR4_OSXSAVE)
> > + /* Safe to read xcr0 */
> > + ghcb_set_xcr0(ghcb, xgetbv(XCR_XFEATURE_ENABLED_MASK));
> > + else
> > + /* xgetbv will cause #UD - use reset value for xcr0 */
> > + ghcb_set_xcr0(ghcb, 1);
> > +
> > + ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0);
> > + if (ret != ES_OK)
> > + return ret;
> > +
> > + if (!(ghcb_rax_is_valid(ghcb) &&
> > + ghcb_rbx_is_valid(ghcb) &&
> > + ghcb_rcx_is_valid(ghcb) &&
> > + ghcb_rdx_is_valid(ghcb)))
> > + return ES_VMM_ERROR;
> > +
> > + leaf->eax = ghcb->save.rax;
> > + leaf->ebx = ghcb->save.rbx;
> > + leaf->ecx = ghcb->save.rcx;
> > + leaf->edx = ghcb->save.rdx;
> > +
> > + return ES_OK;
> > +}
> > +
> > +struct cpuid_ctx {
> > + struct ghcb *ghcb;
> > + struct es_em_ctxt *ctxt;
> > +};
> > +
> > +static void snp_cpuid_ghcb_prot(void *p, struct cpuid_leaf *leaf)
> > +{
> > + struct cpuid_ctx *ctx = p;
> > +
> > + if (__sev_cpuid_ghcb_prot(ctx->ghcb, ctx->ctxt, leaf))
> > + sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_CPUID_HV);
> > +}
> > +
> > static int vc_handle_cpuid_snp(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
> > {
> > + struct cpuid_ctx ctx = { ghcb, ctxt };
> > struct pt_regs *regs = ctxt->regs;
> > struct cpuid_leaf leaf;
> > int ret;
> >
> > leaf.fn = regs->ax;
> > leaf.subfn = regs->cx;
> > - ret = snp_cpuid(ghcb, ctxt, &leaf);
> > + ret = snp_cpuid(snp_cpuid_ghcb_prot, &ctx, &leaf);
> > if (!ret) {
> > regs->ax = leaf.eax;
> > regs->bx = leaf.ebx;
> > diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> > index 89075ff19afa..2cabf617de3c 100644
> > --- a/arch/x86/include/asm/sev.h
> > +++ b/arch/x86/include/asm/sev.h
> > @@ -552,7 +552,8 @@ struct cpuid_leaf {
> > u32 edx;
> > };
> >
> > -int snp_cpuid(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf);
> > +int snp_cpuid(void (*cpuid_hv)(void *ctx, struct cpuid_leaf *),
> > + void *ctx, struct cpuid_leaf *leaf);
> >
> > void __noreturn sev_es_terminate(unsigned int set, unsigned int reason);
> > enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
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