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Message-ID: <fd626425a201655589b33dd8998bb3191a8f0e2f.camel@intel.com>
Date: Tue, 13 May 2025 18:19:56 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
<seanjc@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "Shutemov, Kirill" <kirill.shutemov@...el.com>, "quic_eberman@...cinc.com"
<quic_eberman@...cinc.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
<tabba@...gle.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, "Du, Fan"
<fan.du@...el.com>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "michael.roth@....com"
<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>, "vbabka@...e.cz"
<vbabka@...e.cz>, "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "Yamahata, Isaku"
<isaku.yamahata@...el.com>, "Peng, Chao P" <chao.p.peng@...el.com>,
"Annapurve, Vishal" <vannapurve@...gle.com>, "jroedel@...e.de"
<jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>, "pgonda@...gle.com"
<pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 03/21] x86/virt/tdx: Add SEAMCALL wrapper
tdh_mem_page_demote()
On Thu, 2025-04-24 at 11:04 +0800, Yan Zhao wrote:
> From: Xiaoyao Li <xiaoyao.li@...el.com>
>
> Add a wrapper tdh_mem_page_demote() to invoke SEAMCALL TDH_MEM_PAGE_DEMOTE
> to demote a huge leaf entry to a non-leaf entry in S-EPT. Currently, the
> TDX module only supports demotion of a 2M huge leaf entry. After a
> successful demotion, the old 2M huge leaf entry in S-EPT is replaced with a
> non-leaf entry, linking to the newly-added page table page. The newly
> linked page table page then contains 512 leaf entries, pointing to the 2M
> guest private pages.
>
> The "gpa" and "level" direct the TDX module to search and find the old
> huge leaf entry.
>
> As the new non-leaf entry points to a page table page, callers need to
> pass in the page table page in parameter "page".
>
> In case of S-EPT walk failure, the entry, level and state where the error
> was detected are returned in ext_err1 and ext_err2.
>
> On interrupt pending, SEAMCALL TDH_MEM_PAGE_DEMOTE returns error
> TDX_INTERRUPTED_RESTARTABLE.
>
> [Yan: Rebased and split patch, wrote changelog]
We should add the level of detail here like we did for the base series ones.
>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
> arch/x86/include/asm/tdx.h | 2 ++
> arch/x86/virt/vmx/tdx/tdx.c | 20 ++++++++++++++++++++
> arch/x86/virt/vmx/tdx/tdx.h | 1 +
> 3 files changed, 23 insertions(+)
>
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 26ffc792e673..08eff4b2f5e7 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -177,6 +177,8 @@ u64 tdh_mng_key_config(struct tdx_td *td);
> u64 tdh_mng_create(struct tdx_td *td, u16 hkid);
> u64 tdh_vp_create(struct tdx_td *td, struct tdx_vp *vp);
> u64 tdh_mng_rd(struct tdx_td *td, u64 field, u64 *data);
> +u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *page,
> + u64 *ext_err1, u64 *ext_err2);
> u64 tdh_mr_extend(struct tdx_td *td, u64 gpa, u64 *ext_err1, u64 *ext_err2);
> u64 tdh_mr_finalize(struct tdx_td *td);
> u64 tdh_vp_flush(struct tdx_vp *vp);
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index a66d501b5677..5699dfe500d9 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -1684,6 +1684,26 @@ u64 tdh_mng_rd(struct tdx_td *td, u64 field, u64 *data)
> }
> EXPORT_SYMBOL_GPL(tdh_mng_rd);
>
> +u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *page,
> + u64 *ext_err1, u64 *ext_err2)
> +{
> + struct tdx_module_args args = {
> + .rcx = gpa | level,
This will only ever be level 2MB, how about dropping the arg?
> + .rdx = tdx_tdr_pa(td),
> + .r8 = page_to_phys(page),
> + };
> + u64 ret;
> +
> + tdx_clflush_page(page);
> + ret = seamcall_ret(TDH_MEM_PAGE_DEMOTE, &args);
> +
> + *ext_err1 = args.rcx;
> + *ext_err2 = args.rdx;
How about we just call these entry and level_state, like the caller.
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(tdh_mem_page_demote);
Looking in the docs, TDX module gives some somewhat constrained guidance:
1. TDH.MEM.PAGE.DEMOTE should be invoked in a loop until it terminates
successfully.
2. The host VMM should be designed to avoid cases where interrupt storms prevent
successful completion of TDH.MEM.PAGE.DEMOTE.
The caller looks like:
do {
err = tdh_mem_page_demote(&kvm_tdx->td, gpa, tdx_level, page,
&entry, &level_state);
} while (err == TDX_INTERRUPTED_RESTARTABLE);
if (unlikely(tdx_operand_busy(err))) {
tdx_no_vcpus_enter_start(kvm);
err = tdh_mem_page_demote(&kvm_tdx->td, gpa, tdx_level, page,
&entry, &level_state);
tdx_no_vcpus_enter_stop(kvm);
}
The brute force second case could also be subjected to a
TDX_INTERRUPTED_RESTARTABLE and is not handled. As for interrupt storms, I guess
we could disable interrupts while we do the second brute force case. So the
TDX_INTERRUPTED_RESTARTABLE loop could have a max retries, and the brute force
case could also disable interrupts.
Hmm, how to pick the max retries count. It's a tradeoff between interrupt
latency and DOS/code complexity. Do we have any idea how long demote might take?
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