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td-payload: handle retry result of mapping GPA #501

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merged 1 commit into from
Oct 26, 2023
Merged

td-payload: handle retry result of mapping GPA #501

merged 1 commit into from
Oct 26, 2023

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gaojiaqi7
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Fix: #498

jyao1
jyao1 reviewed Jan 30, 2023
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td-payload/src/arch/x86_64/dma.rs Outdated

use super::paging::{clear_shared_bit, set_shared_bit};

const MAP_GPA_MAX_RETRIES: u8 = 3;
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why retry 3 times?

@gaojiaqi7
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Screenshot 2023-03-20 212728

@gaojiaqi7
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We need to keep retrying without a limit

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Answer from KVM team:

  1. Under what circumstances VMM may return this result?

When the specified range is too large. VMM can't complete the request in practical time. e.g. hundreds of GB.

It's up to VMM implementation.

  1. The spec says guest TD must retry this operation when TDG.VP.VMCALL_RETRY is returned. Should guest TD set any maximum retries or timeout? or it should try again unconditionally, no matter how many times it has already tried?

Retry unconditionally. GHCI spec doesn't have any guide on the retry because it's attack surface for (malicious) VMM against guest TD.

If you want guest to implement guard, it's totally up to you how to protect it. probably it's heuristic protection.

@gaojiaqi7 gaojiaqi7 marked this pull request as draft May 24, 2023 01:40
@gaojiaqi7
tdx-tdcall: handle the retry result of mapgpa
41e900c 
According to GHCI specification, if `TDG.VP.VMCALL_RETRY` is returned,
TD must retry the mapping for the pages in the region starting at the
GPA specified in r11.

Signed-off-by: Jiaqi Gao <jiaqi.gao@intel.com>
@gaojiaqi7 gaojiaqi7 marked this pull request as ready for review October 23, 2023 07:32
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mxu9 commented Oct 25, 2023

It looks good to me.

@jyao1 jyao1 merged commit 3f18d9f into confidential-containers:main Oct 26, 2023
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Need to retry TDG.VP.VMCALL<MapGPA> if the error code is TDG.VP.VMCALL_RETRY
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@gaojiaqi7 @mxu9 @jyao1